space but continue to use CAM16-UCS for distances and updating
centroid positions, before mapping back to the nearest legal 12-bit
RGB position.
Needs some more work to deal with the fact that now that there are
discrete distances (but no fixed minimum) between allowed centroid
positions, the previous notion of convergence doesn't apply. Actually
the centroids can oscillate between positions.
There is room for optimization but this is already reasonably
performant, and the image quality is much higher \o/
kris