fix(image): improve anti-aliasing for LaTeX math expressions

[sei40kr]

Description

Improves anti-aliasing quality for LaTeX math expressions on both light and dark backgrounds by using background-aware rendering during ImageMagick conversion.

Previously, LaTeX math was rendered with a transparent background, causing ImageMagick to default to white for anti-aliasing. This created white halos around math expressions on dark backgrounds.

The fix uses fixed background colors (#ffffff for light, #000000 for dark) based on vim.o.background, then makes the background transparent after proper anti-aliasing is applied.

Related Issue(s)

• Fixes bug: LaTeX math expressions have white halos on dark backgrounds #2644

Screenshots

before:

after:

[sei40kr]

• It is unclear why anti-aliasing processing assuming a white background occurs even when the background is made transparent.
• If you don't mind the reusability of the image cache,

  convert = {
    magick = {
      math = function()
        local bgcolor = Snacks.util.color("Normal", "bg")
        -- stylua: ignore start
        return {
          "-density", 192,
          "-background", bgcolor,
          "{src}[{page}]",
          "-flatten",
          "-fuzz", "2%",
          "-transparent", bgcolor,
          "-trim",
        }
        -- stylua: ignore end
      end,
    },
  },

  yields better results.