OnWillRenderObject はカメラからオブジェクトが可視状態の場合、各カメラごとに一回呼び出されます。
This function is called if the object is deemed visible from the current camera after the culling process.
The method is useful if you need a preparation step for every camera that is rendering the object.
One example of this is rendering reflection onto a render texture.
The reflection will be different for every camera's view point, and needs to be rendered before the original object.
If the object gets culled away by a given camera, the reflections for that object will be skipped.
For usage in a proper context, see the script Water.cs in Assets->Import Package->Effects.
Note that Camera.current will be set to the camera that will render the object.
Also, this is called multiple times per frame.