Xephyr: Check screen resources creation success

If the screen pixmap or the corresponding texture creation with glamor
fails, exit cleanly with an error message instead of segfaulting.

Fixes: https://bugzilla.redhat.com/1431633


Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
(cherry picked from commit b0ce1d08)