util: Tiny optimisation for the linear→srgb conversion
util: Tiny optimisation for the linear→srgb conversion

When converting 0.0 it would be nice if it didn't do any arithmetic.

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler