从 controlID 获取状态对象。
This will return a recycled state object that is unique for controlID.
If there is no state object then a new one will be created and hooked up to the controlID.
On the first call into GetStateObject a new state object will be created.
The controlID uniquely refers to this object. On subsequent calls the stored object will be returned.
另请参阅：GUIUtility.QueryStateObject。