Analogous to `ClutterDrawDebugFlag` but intended for concepts that
are not present in Clutter, such as Wayland/X11 opaque regions.
Also add the first flag for the later.
To set the flag, run:
`Meta.add_debug_paint_flag(Meta.DebugPaintFlag.OPAQUE_REGION)`
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1372