ソートモードの透明オブジェクト。
By default, perspective Cameras sort objects based on distance from Camera position
to the object center; and orthographic Cameras sort based on distance along the view direction.
If you're making a 2D game with a perspective Camera, you might want to use TransparencySortMode.Orthographic
sort mode so that objects are sorted based on distance along the Camera's view.
参考： TransparencySortMode enum、opaqueSortMode