xwayland: Update screen pixmap on output resize

Running Xwayland non-rootless and resizing the output would lead to a
crash while trying to update the larger areas of the root window.

Make sure we resize the backing pixmap according to the new output size
to avoid the crash.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Closes: https://gitlab.freedesktop.org/xorg/xserver/issues/834


Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
(cherry picked from commit ce9455b5)