In rare cases - notably on macOS, because of multiple GL contexts - the lack of a sync point was causing overlays
to disappear for a frame after being redrawn, or sometimes not appear at all. This change makes sure that the display
in one GL context will be correctly synchronised with the other GL context where the overlay texture was uploaded.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5610>
