xwayland/eglstream: Force glFinish() with EGLstream

The buffer we attach to the surface with EGLstream might not be fully
rendered after post damage.

Set the flag to instruct xwl_screen_post_damage() to call glFinish()
once all damage are posted.

This is definitely a big hammer, but a glFlush() is not sufficient to
get rid of the visual glitches.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Closes: xorg/xserver#1171
5 jobs for issue1171 in 4 minutes and 57 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed debian-buster #9656997

00:00:11

 
  Build And Test
passed autotools #9656998

00:04:45

passed meson #9656999

00:02:26

passed meson-noglamor #9657000

00:01:59

passed mingw-cross-build #9657001

00:01:05