Michel Dänzer authored Jan 30, 2020 and Michel Dänzer committed Jan 31, 2020

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>