mesa: Also update the color draw buffer if it's explicitly set to GL_NONE.
mesa: Also update the color draw buffer if it's explicitly set to GL_NONE.

NOTE: This is a candidate for the 7.10 branch.

Signed-off-by: Henri Verbeet <fa9c81d52bf7e42f4eca779fafc7ac8cc1c787d8@gmail.com>

jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,adobe/glsl2agal,mapbox/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,mcanthony/glsl-optimizer,adobe/glsl2agal,zz85/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,adobe/glsl2agal,benaadams/glsl-optimizer,adobe/glsl2agal,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,KTXSoftware/glsl2agal,benaadams/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,tokyovigilante/glsl-optimizer,adobe/glsl2agal,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,zeux/glsl-optimizer,KTXSoftware/glsl2agal,tokyovigilante/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,wolf96/glsl-optimizer,bkaradzic/glsl-optimizer,djreep81/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,mapbox/glsl-optimizer,KTXSoftware/glsl2agal,KTXSoftware/glsl2agal,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,mapbox/glsl-optimizer,wolf96/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,KTXSoftware/glsl2agal,zeux/glsl-optimizer,zz85/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer