Now that ClutterActor has a convenient API for getting the stage views
an actor is presented on, we can remove a large part of the code for
resource-scale calculation and instead rely on the stage-views list.
The way this works is a bit different from the old resource scales:
clutter_actor_get_resource_scale() always returns a scale, but this
value is only guaranteed to be correct when called from a vfunc_paint()
implementation, in all other cases the value is guessed using the scale
of the parent actor or the last valid scale. Now in case the value
previously reported by clutter_actor_get_resource_scale() turns out to
be wrong, "resource-scale-changed" will be emitted before the next paint
and the actor has a chance to update its resources.
The general idea behind this new implementation is for actors which only
need the scale during painting to continue using
clutter_actor_get_resource_scale() as they do right now, and for actors
which need the resource scale on other occasions, like during size
negotiation, to use the scale reported by
clutter_actor_get_resource_scale() but also listen to the
"resource-scale-changed" signal to eventually redo the work using the
correct scale.
The "guessing" of the scale is done with the intention of always giving
actors a scale to work with so they don't have to fall back to a scale
value the actor itself has to define, and also with the intention of
emitting the "resource-scale-changed" signal as rarely as possible, so
that when an actor is newly created, it won't have to load its resources
multiple times.
The big advantage this has over the old resource scales is that it's now
safe to call clutter_actor_get_resource_scale() from everywhere (before,
calling it from size negotiation functions would usually fail). It will
also make it a lot easier to use the resource scale for complex cases
like ClutterText without risking to get into relayout loops.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1276
4a72ac0516