The buffer we attach to the surface with EGLstream might not be fully rendered when resizing windows.
Make sure the previous EGL commands are complete before returning from xwl_glamor_eglstream_post_damage().
This is definitely a big ugly hammer, but a glFlush() is not sufficient to get rid of the visual glitches.
Closes: #1171 (closed)
Goes on top/after !646 (merged)