st/mesa: Do GL_RGBA->GL_RGB texsubimage on hardware
st/mesa: Do GL_RGBA->GL_RGB texsubimage on hardware

State tracker currently backs GL_RGB textures with RGBA almost always.
This means we need to maintain A==1 in these textures to give correct GL_RGB
sampling results.

This change offloads the RGBA->RGB copy to hardware using the new writemask
version of u_blit_pixels.

More src/dstLogical/dstActual triples could be shifted to hardware by
this technique in future patches.

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