llvmpipe: fix sprite texcoord setup for non-projective texturing

llvmpipe: fix sprite texcoord setup for non-projective texturing

Normally the Mesa state tracker uses TXP instructions for texturing.
But if a fragment shader uses texture2D() that's a TEX instruction.
In that case we were incorrectly computing the texcoord coefficients
in the point sprite setup code.  Some new comments in the code explain
things.