// ------------------------------------------------------------------
// 遍历_go及所有子sprite，按深度优先更新它们的globalDepth并且刷新mesh
// ------------------------------------------------------------------
private float GetParentGlobalDepth (GameObject _go) {
exLayeredSprite parent = _go.GetComponentUpwards<exLayeredSprite>();
return parent != null ? (parent as IFriendOfLayer).globalDepth : 0.0f;
}