st/mesa: make generic CopyPixels path work with MSAA visuals

st/mesa: make generic CopyPixels path work with MSAA visuals

We have to use pipe->blit, not resource_copy_region, so that the read buffer
is resolved if it's multisampled. I also removed the CPU-based copying,
which just did format conversion (obsoleted by the blit).

Also, the layer/slice/face of the read buffer is taken into account (this was
ignored).

Last but not least, the format choosing is improved to take float and integer
read buffers into account.

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