nvfx: new 2D: use a CPU copy for up to 4 pixels, up from 0
nvfx: new 2D: use a CPU copy for up to 4 pixels, up from 0

Seems a reasonable threshold for now.

Significantly speeds up Piglit's 1x1 glReadPixels (but, you know,
reading pixels in 1x1 blocks is NOT a good idea, especially if you
might be running on a less-than-perfect driver).

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