immediately call onFrameChange() when starting spritesheet animation
tweenScale() is uniform
toFront -> sendToForeground() -> calls node.setLast()
toBack() -> sendToBackground() -> calls node.setFirst()
tween alpha
track numActiveTweens
add TweenBuilder
also free all controllers attached to a spatial node when calling spatial.free()
add RepeatType to tweening
ignore spatial if composite is null
add centerPivot[X,Y], centerOrigin[X,Y]
return a copy of the viewport rectangle