st/mesa: implement glBitmap unpacking from a PBO, for the cache path
st/mesa: implement glBitmap unpacking from a PBO, for the cache path

We weren't mapping the PBO when using the bitmap cache (but we had
the PBO code for the non-cache path.)

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=61026

Note: This is a candidate for the stable branches.

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