This is to be used with gst_video_overlay_set_render_rectangle()
so the application can calculate a rectangle that fits inside
the display. The property changes are notify in a way that you
can watch either notify::display-width or notify::display-height
and both will be up-to-data when this is called back. Before the
element is started, the size will be 0x0.
https://bugzilla.gnome.org/show_bug.cgi?id=784599
737067e793