vc4: Don't worry about partial Z/S clear if the other is already cleared.
vc4: Don't worry about partial Z/S clear if the other is already cleared.

We have to be careful to not smash the value they're clearing to, but
other than that we're fine.  Avoids quad clears in Processing, which likes
to do glClear(Z|S); glClear(Z).

Improves performance of Processing's QuadRendering demo at 5000 quads by
5.46507% +/- 1.35576% (n=15 before, 32 after)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler