i965: Enable the accelerated ReadPixels path on gen4 along with pre-gen4.
i965: Enable the accelerated ReadPixels path on gen4 along with pre-gen4.

Passes piglit pbo-read-argb8888, and doesn't otherwise regress quick.tests.

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