draw: handle more TGSI_SEMANTIC_COLOR indices

draw: handle more TGSI_SEMANTIC_COLOR indices

It could only handle indices 0/1, otherwise what happened was bad (accessing
array out of bounds, no crash but kind of random). This is enough for the gl
state tracker (primary/secondary color) but not enough for some other state
trackers (d3d9 has no limits on the number of color interpolants).
The complexity with color semantics are all due to the front/back mapping (2
outputs in the vs map to one input in the fs) so this isn't extended to
indices > 1 - d3d9 has no use for back colors, therefore this isn't needed and
still only 2 back colors can be handled correctly.

Reviewed-by: Brian Paul <brianp@vmware.com>