mesa: Add gl_MESAFogParamsOptimized for our special pre-computed fog params.

mesa: Add gl_MESAFogParamsOptimized for our special pre-computed fog params.

It would be nice if we handled optimized uniform math like this in
some generic way, since people often end up doing uniform expressions
in shaders, but for now keep this hard-coded like it was in the
texenvprogram code.