Registry class to keep track of all IClippers that exist in the scene

This is used during the CanvasUpdate loop to cull clippable elements. The clipping is called after layout, but before Graphic update.

实现和CanvasUpdateRegistry类似，一个m_Clippers的IndexedSet<IClipper>，保存可裁剪对象。目前专门给RectMask2D用的

在layout更新后，Graphic更新前执行

被RectMask2D调用，在OnEnable将自身添加进数组，OnDisable时反注册

不同的是，m_Clippers不会主动清理