世界坐标转屏幕坐标，
目的
UI层的物体能跟随3D世界的物体
应用场景
血条，聊天气泡，物体小提示

转换坐标

        private Vector2 GetScreenPosition(Vector3 worldPosition){Vector2 anchoredPosition = _camera.WorldToScreenPoint(worldPosition);anchoredPosition *= CanvasHelper.RootCanvasSize.x / Screen.width;float newWidth = anchoredPosition.x - CanvasHelper.RootCanvasSize.x * 0.5f;float newHeight = anchoredPosition.y - CanvasHelper.RootCanvasSize.y * 0.5f;return new Vector2(newWidth, newHeight);}

在update里更新

  		public void OnUpdate(){if (_target != null && CharacterController.Instance != null){rectTransform.anchoredPosition = GetScreenPosition(_target.position);}}