Add colour quantization to convert 16/24/32-bit RGB to 8-bit indexed images

[kaixiong]

I'm not sure how relevant colour quantization is now for today's display devices, but this would be great for completeness sake.

There are some well known and performant algorithms to quantize colours to 256. Relatively easy to implement too:

Octrees

• Dr Dobb's Journal (this brings me back to high school!)
• Leptonica

Median Cut

• Wikipedia