imshow fails with Sixel backend: MethodError in sixel_encode

Hello! I got this error when trying the readme example. I'm using wezterm on linux, which has sixel support. Here is how to reproduce the error:
```
julia> using Pkg

julia> Pkg.status("ImageInTerminal")
No Matches in `~/.julia/environments/v1.12/Project.toml`

julia> Pkg.status("Sixel")
No Matches in `~/.julia/environments/v1.12/Project.toml`

julia> Pkg.add("ImageInTerminal")
   Resolving package versions...
   Installed ImageInTerminal ─ v0.5.4
    Updating `~/.julia/environments/v1.12/Project.toml`
  [d8c32880] + ImageInTerminal v0.5.4
    Updating `~/.julia/environments/v1.12/Manifest.toml`
  [a8cc5b0e] + Crayons v4.1.1
  [d8c32880] + ImageInTerminal v0.5.4
  [c8c2cc18] + XTermColors v0.2.1
Precompiling packages finished.
  1 dependency successfully precompiled in 2 seconds. 232 already precompiled.

julia> Pkg.add("Sixel")
   Resolving package versions...
   Installed Sixel ─ v0.1.5
    Updating `~/.julia/environments/v1.12/Project.toml`
  [45858cf5] + Sixel v0.1.5
    Manifest No packages added to or removed from `~/.julia/environments/v1.12/Manifest.toml`

julia> Pkg.status("ImageInTerminal")
Status `~/.julia/environments/v1.12/Project.toml`
  [d8c32880] ImageInTerminal v0.5.4

julia> Pkg.status("Sixel")
Status `~/.julia/environments/v1.12/Project.toml`
  [45858cf5] Sixel v0.1.5

julia> using TestImages

julia> testimage("mandril_color")
512×512 Matrix{ColorTypes.RGB{FixedPointNumbers.N0f8}}:
 RGB(0.651, 0.545, 0.243)  RGB(0.22, 0.188, 0.137)   …  RGB(0.671, 0.741, 0.443)
 RGB(0.416, 0.451, 0.196)  RGB(0.545, 0.392, 0.169)     RGB(0.459, 0.486, 0.224)
 RGB(0.294, 0.251, 0.125)  RGB(0.482, 0.42, 0.188)      RGB(0.38, 0.29, 0.157)
 RGB(0.227, 0.29, 0.176)   RGB(0.541, 0.518, 0.212)     RGB(0.196, 0.29, 0.165)
 RGB(0.384, 0.286, 0.165)  RGB(0.549, 0.482, 0.2)       RGB(0.282, 0.325, 0.157)
 RGB(0.286, 0.29, 0.125)   RGB(0.282, 0.294, 0.125)  …  RGB(0.671, 0.741, 0.451)
 RGB(0.153, 0.09, 0.09)    RGB(0.204, 0.153, 0.09)      RGB(0.612, 0.612, 0.471)
 RGB(0.153, 0.063, 0.094)  RGB(0.188, 0.169, 0.094)     RGB(0.553, 0.612, 0.294)
 RGB(0.192, 0.165, 0.196)  RGB(0.224, 0.129, 0.09)      RGB(0.459, 0.478, 0.227)
 RGB(0.169, 0.098, 0.09)   RGB(0.153, 0.063, 0.094)     RGB(0.537, 0.514, 0.224)
 RGB(0.09, 0.094, 0.071)   RGB(0.188, 0.161, 0.098)  …  RGB(0.286, 0.2, 0.11)
 RGB(0.161, 0.063, 0.086)  RGB(0.192, 0.157, 0.098)     RGB(0.196, 0.29, 0.165)
 ⋮                                                   ⋱
 RGB(0.486, 0.643, 0.541)  RGB(0.459, 0.58, 0.518)      RGB(0.349, 0.259, 0.196)
 RGB(0.447, 0.58, 0.514)   RGB(0.443, 0.541, 0.518)     RGB(0.251, 0.22, 0.263)
 RGB(0.349, 0.416, 0.486)  RGB(0.447, 0.482, 0.502)     RGB(0.29, 0.325, 0.322)
 RGB(0.388, 0.482, 0.42)   RGB(0.451, 0.514, 0.412)     RGB(0.255, 0.192, 0.192)
 RGB(0.349, 0.416, 0.478)  RGB(0.392, 0.478, 0.427)  …  RGB(0.294, 0.318, 0.318)
 RGB(0.388, 0.38, 0.384)   RGB(0.345, 0.412, 0.482)     RGB(0.255, 0.294, 0.263)
 RGB(0.322, 0.357, 0.29)   RGB(0.349, 0.416, 0.478)     RGB(0.286, 0.294, 0.251)
 RGB(0.353, 0.416, 0.478)  RGB(0.443, 0.514, 0.514)     RGB(0.294, 0.314, 0.329)
 RGB(0.455, 0.541, 0.631)  RGB(0.451, 0.58, 0.506)      RGB(0.29, 0.294, 0.224)
 RGB(0.451, 0.588, 0.667)  RGB(0.455, 0.537, 0.62)   …  RGB(0.251, 0.224, 0.251)
 RGB(0.451, 0.588, 0.667)  RGB(0.455, 0.537, 0.62)      RGB(0.251, 0.224, 0.251)

julia> using ImageInTerminal

julia> testimage("mandril_color")
Error showing value of type Matrix{ColorTypes.RGB{FixedPointNumbers.N0f8}}:

SYSTEM (REPL): showing an error caused an error
ERROR: 1-element ExceptionStack:
MethodError: no method matching sixel_write_callback_function(::Ptr{Int8}, ::Int32, ::IOContext{IOBuffer})
The function `sixel_write_callback_function` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  sixel_write_callback_function(::Ptr{Int8}, ::Int32, ::Ref{T}) where T<:IO
   @ Sixel ~/.julia/packages/Sixel/X78VX/src/backend/libsixel/encoder.jl:14

Stacktrace:
  [1] sixel_encode
    @ ~/.julia/packages/Sixel/X78VX/src/backend/libsixel/CModule.jl:286 [inlined]
  [2] (::Sixel.LibSixel.LibSixelEncoder)(io::IOContext{IOBuffer}, bytes::Matrix{ColorTypes.RGB{FixedPointNumbers.N0f8}})
    @ Sixel.LibSixel ~/.julia/packages/Sixel/X78VX/src/backend/libsixel/encoder.jl:34
  [3] sixel_encode(io::IOContext{…}, img::Matrix{…}, enc::Sixel.LibSixel.LibSixelEncoder; transpose::Bool, kwargs::@Kwargs{})
    @ Sixel ~/.julia/packages/Sixel/X78VX/src/encoder.jl:93
  [4] sixel_encode (repeats 2 times)
    @ ~/.julia/packages/Sixel/X78VX/src/encoder.jl:77 [inlined]
  [5] sixel_encode
    @ ~/.julia/packages/ImageInTerminal/U2jcL/src/ImageInTerminal.jl:131 [inlined]
  [6] imshow(io::IOContext{REPL.LimitIO{…}}, img::Matrix{ColorTypes.RGB{…}}, maxsize::Tuple{Int64, Int64}; kw::@Kwargs{})
    @ ImageInTerminal ~/.julia/packages/ImageInTerminal/U2jcL/src/ImageInTerminal.jl:115
  [7] imshow (repeats 2 times)
    @ ~/.julia/packages/ImageInTerminal/U2jcL/src/ImageInTerminal.jl:110 [inlined]
  [8] show(io::IOContext{REPL.LimitIO{…}}, mime::MIME{Symbol("text/plain")}, img::Matrix{ColorTypes.RGB{…}})
    @ ImageInTerminal ~/.julia/packages/ImageInTerminal/U2jcL/src/ImageInTerminal.jl:68
  [9] show_repl(io::IO, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:548
 [10] show_limited(io::IO, mime::MIME, x::Any)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:517
 [11] (::REPL.var"#display##0#display##1"{REPL.REPLDisplay{…}, MIME{…}, Base.RefValue{…}})(io::Any)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:540
 [12] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:833
 [13] display(d::REPL.REPLDisplay, mime::MIME{Symbol("text/plain")}, x::Any)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:526
 [14] display
    @ ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:546 [inlined]
 [15] display(x::Any)
    @ Base.Multimedia ./multimedia.jl:340
 [16] (::REPL.var"#print_response##2#print_response##3")()
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:590
 [17] repl_backend_loop(backend::REPL.REPLBackend, get_module::Function)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:445
 [18] start_repl_backend(backend::REPL.REPLBackend, consumer::Any; get_module::Function)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:427
 [19] start_repl_backend
    @ ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:424 [inlined]
 [20] run_repl(repl::REPL.AbstractREPL, consumer::Any; backend_on_current_task::Bool, backend::Any)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:653
 [21] run_repl(repl::REPL.AbstractREPL, consumer::Any)
    @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:639
 [22] run_std_repl(REPL::Module, quiet::Bool, banner::Symbol, history_file::Bool)
    @ Base ./client.jl:478
 [23] run_main_repl(interactive::Bool, quiet::Bool, banner::Symbol, history_file::Bool)
    @ Base ./client.jl:499
 [24] repl_main
    @ ./client.jl:586 [inlined]
 [25] _start()
    @ Base ./client.jl:561
Stacktrace:
 [1] print_response(errio::IO, response::Any, backend::Union{…}, show_value::Bool, have_color::Bool, specialdisplay::Union{…})
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:599
 [2] (::REPL.var"#print_response##0#print_response##1"{REPL.LineEditREPL, Pair{Any, Bool}, Bool, Bool})(io::Any)
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:558
 [3] with_repl_linfo(f::Any, repl::REPL.LineEditREPL)
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:833
 [4] print_response(repl::REPL.AbstractREPL, response::Any, show_value::Bool, have_color::Bool)
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:556
 [5] (::REPL.var"#do_respond#73"{…})(s::REPL.LineEdit.MIState, buf::Any, ok::Bool)
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:1192
 [6] run_interface(terminal::Base.Terminals.TextTerminal, m::REPL.LineEdit.ModalInterface, s::REPL.LineEdit.MIState)
   @ REPL.LineEdit ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/LineEdit.jl:2854
 [7] run_frontend(repl::REPL.LineEditREPL, backend::REPL.REPLBackendRef)
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:1663
 [8] (::REPL.var"#61#62"{REPL.LineEditREPL, REPL.REPLBackendRef})()
   @ REPL ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/REPL/src/REPL.jl:650
Some type information was truncated. Use `show(err)` to see complete types.

julia> versioninfo()
Julia Version 1.12.4
Commit 01a2eadb047 (2026-01-06 16:56 UTC)
Build Info:
  Official https://julialang.org release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 24 × 12th Gen Intel(R) Core(TM) i7-12800HX
  WORD_SIZE: 64
  LLVM: libLLVM-18.1.7 (ORCJIT, alderlake)
  GC: Built with stock GC
Threads: 1 default, 1 interactive, 1 GC (on 24 virtual cores)

julia> using Sixel

julia> Sixel.is_sixel_supported()
true

julia> sixel_encode(testimage("mandril_color"))
```
I found that directly using `sixel_encode` renders the image, so it appears that using stdout is fine.
<img width="931" height="1117" alt="Image" src="https://github.com/user-attachments/assets/6932b7ab-88b7-4197-8cef-d637ec200f4e" />

When I check the source code of `imshow`, I noticed that it sends IOContext to `sixel_encode`, which appears to trigger the error.
```
function imshow(
    io::IO, img::AbstractArray{<:Colorant}, maxsize::Tuple=displaysize(io); kw...
)
    buf = IOContext(PipeBuffer(), :color => get(io, :color, false))
    if choose_sixel(img)
        sixel_encode(buf, img)
...
``` 
I changed this part of the code to
```
function imshow(
    io::IO, img::AbstractArray{<:Colorant}, maxsize::Tuple=displaysize(io); kw...
)
    inner_buf = PipeBuffer()
    buf = IOContext(inner_buf, :color => get(io, :color, false))
    if choose_sixel(img)
        sixel_encode(inner_buf, img)
```
Now it appears to work properly

<img width="1184" height="920" alt="Image" src="https://github.com/user-attachments/assets/cacdbfa2-8a4f-4070-b0cf-f6147f0081e4" />

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imshow fails with Sixel backend: MethodError in sixel_encode #79

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imshow fails with Sixel backend: MethodError in sixel_encode #79

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