Implementing `ReadBuffer` for `&mut [C;N]` might be unsound

[jounathaen]

As mentioned in #23, we basically implement ReadBuffer for all &mut [C;N] in

embedded-graphics-framebuf/src/backends.rs

Line 92 in 4205fb2

unsafe impl<'a, C: PixelColor, const N: usize> DMACapableFrameBufferBackend for &'a mut [C; N] {

This might be problematic because the slice could not be suitable for DMA access. Maybe it is already fine as is, but as I haven't fully thought this through, I'll leave this as an issue for now.