Allow gl user interfaces to block display device gl rendering.
The ui code might want to do that in case it takes a little
longer to bring things to screen, for example because we'll
hand over a dma-buf to another process (spice will do that).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>