mesa: Add umbrella features.
mesa: Add umbrella features.

Add FEATURE_GL, FEATURE_ES1, and FEATURE_ES2 for OpenGL, OpenGL ES 1.x,
and OpenGL ES 2.x respectively.  Define individual features through the
new umbrella features.  There is no real change introduced by this
commit.

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