This makes it a bit more reliable to add more reference images, but be sure not to touch any existing ones. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3859>
This allows it to be used in processes that doesn't have a stage view etc, e.g. screen cast clients. Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3859>