mesa: avoid valgrind warning due to opaque only being set sometimes
mesa: avoid valgrind warning due to opaque only being set sometimes

Valgrind complains with a "Conditional jump or move depends on
uninitialised value(s)" warning due to opaque being conditionally
initialized. However in the punchthrough_alpha == true case, it is
always initialized, so just flip the condition around to silence the
warning.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Reviewed-by: Chris Forbes <5dcac483e7a6e31a5cbe89f38cc69dff8ee93dac@ijw.co.nz>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler