st/mesa: use a single memcpy in st_ReadPixels when possible
st/mesa: use a single memcpy in st_ReadPixels when possible

This avoids costly address recomputations, function overhead, and may trigger
large copy optimizations.

Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler