mesa: Add _mesa_swap2_copy and _mesa_swap4_copy

mesa: Add _mesa_swap2_copy and _mesa_swap4_copy

We have _mesa_swap{2,4} but these do in-place byte-swapping only. The new
functions receive an extra parameter so we can swap bytes on a source
input array and store the results in a (possibly different) destination
array.

This is useful to implement byte-swapping in pixel uploads, since in this
case we need to swap bytes on the src data which is owned by the
application so we can't do an in-place byte swap.

v2:
  - Include compiler.h in image.h, which is necessary to build in MSCV as
    indicated by Brian Paul.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>