Intended to be used to pass state from screen cast clients down the
line. The first use case will be a boolean whether a screen cast is a
plain recording or not, e.g. letting the Shell decide whether to use a
red dot as the icon, or the generic "sharing" symbol.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1377
It takes coordinates in stage coordinate space, and will result in
a screen cast stream consisting of that area, but scaled up by the scale
factor of the view that overlaps with the area and has the highest scale
factor.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1207