st/mesa: implement blit-based ReadPixels

st/mesa: implement blit-based ReadPixels

Initial version contributed by: Martin Andersson <g02maran@gmail.com>

This is only used if the memcpy path cannot be used and if no transfer ops
are needed. It's pretty similar to our TexImage and GetTexImage
implementations.

The motivation behind this is to be able to use ReadPixels every frame and
still have at least 20 fps (or 60 fps with a powerful GPU and CPU)
instead of 0.5 fps.

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