Support images with alpha channel

[makew0rld]

Thanks to @/Shrinks99

This will not do dithering in the alpha channel, but it will properly handle images with non-opaque pixels.

• Leave fully transparent pixels as they are
• Partially transparent pixels (example)
  • I have a premult color, a=0.5
  • I take the RGB and dither it, and end up with a palette color (with implicit a=1)
  • I multiply the palette color by 0.5
  • I store the multiplied color with a=0.5

DitherPaletted wouldn't support this, as this would require breaking the guarantee that the returned image has the same palette as the Ditherer.

[makew0rld]

I think the example is missing a step, right after the first one. Premultiplied colors need to be un-premultiplied before dithering, otherwise the dithering will treat the colors as if they're darker.

[makew0rld]

Support for this was added in 979ffde. This issue will be closed after some more testing.

[makew0rld]

Examples are in the repo tree now, here they are directly linked: