mesa: optimize _UsesDualSrc blend flag setting
mesa: optimize _UsesDualSrc blend flag setting

For glBlendFunc and glBlendFuncSeparate(), the _UsesDualSrc flag
will be the same for all buffers, so no need to compute it N times.

Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler