vc4: Convert blending to being done in 4x8 unorm normally.

vc4: Convert blending to being done in 4x8 unorm normally.

We can't do this all the time, because you want blending to be done in
linear space, and sRGB would lose too much precision being done in 4x8.
The win on instructions is pretty huge when you can, though.

total uniforms in shared programs: 32065 -> 32168 (0.32%)
uniforms in affected programs:     327 -> 430 (31.50%)
total instructions in shared programs: 92644 -> 89830 (-3.04%)
instructions in affected programs:     15580 -> 12766 (-18.06%)

Improves openarena performance at 1920x1080 from 10.7fps to 11.2fps.