描述方向的轴，将沿此方向测量对象距离，从而进行排序。
This is the default axis used by the rendering pipeline for sorting in TransparencySortMode.CustomAxis mode for all Cameras including the SceneView's. You may override this for each invididual Camera by calling Camera.transparencySortAxis and Camera.transparencySortMode APIs.
See Also: TransparencySortMode enum, Camera.transparencySortAxis, Camera.transparencySortMode.