st/mesa: rework blit_copy_pixels to use pipe->blit

st/mesa: rework blit_copy_pixels to use pipe->blit

There were 2 issues with it:
- resource_copy_region doesn't allow different sample counts of both src
  and dst, which can occur if we blit between a window and a FBO, and
  the window has an MSAA colorbuffer and the FBO doesn't.
  (this was the main motivation for using pipe->blit)
- blitting from or to a non-zero layer/slice/face was broken, because
  rtt_face and rtt_slice were ignored.

blit_copy_pixels is now used even if the formats and orientation of
framebuffers don't match.

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