added progress accessor to Tween

find a Tween's progress, as determined by its transition function, from
0 to 1. previously, this was calculated only within animate() (and was
inefficiently calculated separately for each tweened property).

useful for updating things in a Tween's onUpdate() callback in sync with the Tween's progress, when it's not possible for the Tween to do the update itself.  (e.g. update a Sprite's filter values)