Since we now always return a resource scale, we can remove the boolean
return value from clutter_actor_get_resource_scale() and
_clutter_actor_get_real_resource_scale(), and instead simply return the
scale.
While at it, also remove the underscore from the
_clutter_actor_get_real_resource_scale() private API.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1276
And add the necessary glue so those initialize a X11 clutter backend.
This should get Clutter tests that are dependent on windowing to work
again, thus they were enabled back again.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/672
