Start an eager tween associated with this entity. This is
the same as a regular tween, except that it will start updating
itself using current frame's delta time instead of next frame like in a regular tween.
Parameters:
easing
The easing to use
duration
The duration of the tween
fromValue
The start value of the tween
toValue
The end value of the tween
update
An update function called at each iteration of the tween