检查代码块中是否有任何控件被更改。
When needing to check if GUI.changed is set to true inside a block of code,
wrap the code inside BeginChangeCheck () and EndChangeCheck () like this:
EndChangeCheck will only return true if GUI.changed was set to true
inside the block, but GUI.changed will be true afterwards both if it
was set to true inside and if it was already true to begin with.