vc4: Restructure color packing as a series of channel replacements.

vc4: Restructure color packing as a series of channel replacements.

I'm using this in some WIP commits for doing blending in 8888 instead of
vec4.  But it also gives us these results immediately, thanks to allowing
more uniforms/immediates in the arguments:

total instructions in shared programs: 41027 -> 40960 (-0.16%)
instructions in affected programs:     4381 -> 4314 (-1.53%)