Allow scenes to be activated as soon as it is ready.
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. While isDone is false, the AsyncOperation queue is stalled. For example, if a LoadSceneAsync.allowSceneActivation is set to false, and another AsyncOperation (e.g. SceneManager.UnloadSceneAsync ) is initialized, the last operation will not be called before the first allowSceneActivation is set to true.