swr: rework vert <-> frag shader linkage logic

swr: rework vert <-> frag shader linkage logic

Fixes a few things:
 - sprite coords only apply to generic varyings, and are a bitmask
 - back color only applies in 2-sided lighting mode
 - handle some odd situations between only some front/back colors being
   there. This is only semi-legal in GL, but we shouldn't start
   crashing.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>