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
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