mesa: Convert fixed function fragment program generator to GLSL IR.

mesa: Convert fixed function fragment program generator to GLSL IR.

This is a step towards providing a direct route for drivers accepting
GLSL IR for codegen.  Perhaps more importantly, it runs the fixed
function fragment program through the GLSL IR optimization.  Having
seen how easy it is to make ugly fixed function texenv code that can
do unnecessary work, this may improve real applicatinos.