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: xorg/xserver#834
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
3 jobs for issue834 in 12 minutes and 15 seconds (queued for 31 seconds)