允许在场景准备就绪后立即激活场景。
当与 LoadLevelAsync 和 LoadLevelAdditiveAsync 结合使用时，它允许您
延时场景的实际激活。（并卸载前一个场景）。
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.