A very simple full-color to indexed-color converter, based on the usual
k-means algorithm.

I wrote it because I couldn't offhand find anything that dealt with alpha
in a better fashion than just converting it to 1-bit depth, and I needed
something that did. (This one works in premultiplied alpha, since that's
really the only thing that makes sense. The distance function could use
some work, though.) Apart from that, there's nothing very sophisticated
about it.