The Animatable interface implementation will always have the computed
value applied, whilst the non-Animatable objects go through the
interval validation first to avoid incurring in assertions and
warnings.
The Animatable::animate_property() should also be able to validate the
property it's supposed to interpolate, and eventually discard it. This
requires adding a return value to the virtual function (and its wrapper
function).
The Animation code will then apply the computed value only if the
animate_property() returns TRUE -- unifying the code path with the
non-Animatable objects.
The ClutterAnimatable interface is meant to be used by GObject
classes to override the value computation for an animatable
property within the boundaries of an interval.
It is composed of a single virtual function, animate_property();
its implementation will receive the ClutterAnimation used to
animate the object; the property name; the initial and final
interval values; and the progress factor as retrieved by the
Alpha object bound to the Animation instance.
