Typically, to stream the content of a window, we need a way to copy the
content of its window-actor into a buffer, transform relative input
coordinates to relative position within the window-actor and a mean to
get the window bounds within the buffer.
For this purpose, add a new GType interface `MetaScreenCastWindow` with
the methods needed for screen-cast window mode:
* meta_screen_cast_window_get_buffer_bounds()
* meta_screen_cast_window_get_frame_bounds()
* meta_screen_cast_window_transform_relative_position()
* meta_screen_cast_window_capture_into()
This interface is meant to be implemented by `MetaWindowActor` which has
access to all the necessary bits to implement them.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/306
