st/mesa: simplify and improve CopyTexSubImage

st/mesa: simplify and improve CopyTexSubImage

It has become a bit messy.

Changes:

- finally correct checking for transfer ops depending on the base format

- making sure the base internal format and the texture format match
  (we were ignoring it, but it's important for correctness)

- the way-too-strict rule that both src and dst base formats must be the same
  was dropped; ensuring the simpler and more permissive rule mentioned above
  is enough

- stop using util_blit_pixels; pipe->blit is flexible enough, and now that we
  have RGBX and red-alpha formats, pipe->blit can be used for more cases

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