ui/gtk: Support shared surface for most pixman formats

At least all the ones I've tested. We make the assumption that
pixman is going to be better at conversion than we are.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

[ kraxel: just hook up qemu_pixman_check_format ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>