vaapisink: don't mask button events for foreign windows
vaapisink: don't mask button events for foreign windows

Don't subscribe to button press events when using a foreing window,
because the user created window would trap those events, preveting the
show of frames.

https://bugzilla.gnome.org/show_bug.cgi?id=791615

GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi