By checking if the interval is valid inside compute_value() we can catch
the cases where the interval values of a PropertyTransition are set
after the transition has been added to an Animatable instance - i.e. the
following code:
let transition = new Clutter.PropertyTransition();
transition.set_property_name('opacity');
actor.add_transition('opacityAnim', transition);
transition.set_to_value(0);
should be equivalent to:
let transition = new Clutter.PropertyTransition();
transition.set_property_name('opacity');
transition.set_to_value(0);
actor.add_transition('opacityAnim', transition);
instead of emitting a warning.
Once a ClutterPropertyTransition is attached to a ClutterAnimatable, if
no interval is set we can simply use the current state of the property
to define the from and to values. This allows the creation of property
transitions from the current state of the Animatable instance without
excessive verbosity.
Should not have been there in the first place: the animatable will be
set either using ClutterTransition API, or when adding the transition
to a ClutterActor.
