mesa: fix texenv generation when num color bufs == 0
mesa: fix texenv generation when num color bufs == 0

Before, if there were no color buffers enabled (with glDrawBuffers(GL_NONE))
when the texenv program was generated, we'd emit writes to OUTPUT[1] but
the OutputsWritten mask was 0.  This inconsistency caused an assertion to
fail later in the Mesa->TGSI translation.

Fixes fd.o bug 28169

NOTE: this is a candidate for the 7.8 branch (and depends on commit
b6b9b17d27c570cc99ae339e595cf2f63ca5e8d7).

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