mesa: add common format-independent memcpy-based ReadPixels path

mesa: add common format-independent memcpy-based ReadPixels path

I'll need the _mesa_readpixels_needs_slow_path function for the blit-based
version, but it's also useful to have this memcpy-based path in one place
and not scattered across several functions.

v2: add "const" to function parameters

Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Brian Paul <brianp@vmware.com>