st/mesa: 'fix' point coord semantic info

st/mesa: 'fix' point coord semantic info

This fixes the progs/glsl/pointcoord.c demo.  But this isn't a proper fix.
We really need a TGSI_SEMANTIC_POINT_COORD label so that the draw module
can determine which fragment input / vertex output slot needs to be set
up with the point coordinate info.  We've been using generic slot 0 so far.

This would also require telling the draw module about fragment shaders
(something it doesn't have at this time).