MetaBackend owns MetaScreenCast which owns MetaScreenCastSession which owns MetaScreenCastStream. Make it possible to fetch objects in the oppositev direction too. https://gitlab.gnome.org/GNOME/mutter/merge_requests/357
Window mode will cast the content of a single window using the `MetaScreenCastWindow` interface. https://gitlab.gnome.org/GNOME/mutter/merge_requests/306