シーンが準備完了となったタイミングですぐにシーンが有効化されることを許可します。
When used in combination with LoadLevelAsync & LoadLevelAdditiveAsync it allows you
to delay the actual activation of the scene. (And unloading of the previous scene).
When allowSceneActivation is set to false then progress is stopped at 0.9. The isDone is then maintained at false. When allowSceneActivation is set to true isDone can complete.