avoiding prepare-xwindow-id and gdk threads

[Dan]

OK, thanks again. One last question: how urgent is it for the app to
take action on receiving prepare-xwindow-id? What does gstreamer do if
I don't call imagesink.set_xwindow_id() "soon enough"? Could I call
set_xwindow_id() from an idle_add() callback(*)?

(*) which *seems* to schedule events for the main thread, though I can
find no definitive either GTK or PyGTK documentation stating this.