Improvement of True color image quantization to 256 colors.
see http://pulkomandy.tk/projects/GrafX2/ticket/26 I noticed 2 issues with the existing code : Cluster splitting algorithm and cluster sorting (to decide which cluster to sort).
I added another cluster splitting algorithm (in 2 cluster of half volume instead of half population) and another cluster sort (by diagonal of the "color cube" instead of its volume)
See the results :