xwayland/glamor-gbm: Add xwl_glamor_gbm_post_damage hook

It flushes any pending drawing to the kernel, to make sure it'll be
visible to the Wayland server.

Without this, it was possible for the Wayland server to process surface
commits before Xwayland got around to flushing the corresponding
drawing, which could result in stale or even completely random window
contents being visible.

v2:
* Make EGL backend post_damage hook mandatory, don't check for NULL in
  xwl_glamor_post_damage. (Olivier Fourdan)

Closes: xorg/xserver#951
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
3 jobs for xwl_glamor_gbm_post_damage in 10 minutes and 54 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Docker Image
passed #1521107
debian-testing

00:00:33

 
  Build And Test
passed #1521108
autotools-build-and-test

00:10:21

passed #1521109
meson-build-and-test

00:03:46