拖动UI，3D物体跟随

            Vector3 CO_Direction = mFollowSelectGo.transform.position - MainCamera.transform.position;//相机到物体的位置向量
            float cPlane = Vector3.Dot(CO_Direction, MainCamera.transform.forward);//得到摄像机与物体所在平面的距离
            float CO_Distance = MainCamera.WorldToScreenPoint(mFollowSelectGo.transform.position).z;//相机到物体的世界坐标距离

            Vector2 screenPos = RectTransformUtility.WorldToScreenPoint(UICamera, UI_Image.transform.position);
            mFollowSelectGo.transform.position = MainCamera.ScreenToWorldPoint(new Vector3(screenPos.x, screenPos.y, CO_Distance)); //CO_Distance 等价与 cPlane