BeginChangeCheck と EndChangeCheck で囲んだ GUI グループ内の GUI 要素でなんらかの変更がなされたときにアクションを起こすために使用されます。
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.