xcb: Optimize non-shm backing store flushing

Unfortunately the functions in xcb-image are quite slow, both for the
subimage generation and the byte order swapping. xcb_image_subimage is
implemented as a pixel by pixel copy, and the xcb byte swapping is done
manually without utilizing potential CPU instructions to accelerate the
swap.

Replace both with their Qt equivalents.

Change-Id: I1fe1fe5d9576fdf2bab4a8c401d2a6bb842c2727
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>