斯凯利 制作骨架动画的箱子。 （可选）提供反向运动学功能。 例子 // build a skelly with leg and two arms. let mut skelly = Skelly :: :: new (); let foot = skelly. add_root (Point3 :: origin ()); let leg = skelly. attach (Vector3 :: z (). into (), foot); let waist = skelly. attach