property-transition: Verify the interval on compute_value()

property-transition: Verify the interval on compute_value()

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.