Improved the code in SceneStage.

[NEW] Added Viewport.dispose().
[NEW] Added Viewport.isDisposed().
[NEW] Added Viewport.isVisible().
[NEW] Now Viewport can construct with z parameter.
[ENH] Improved the code in SceneStage.
[MOD] Now Graphics.update() would check the existence of viewport and
its visibility.
[FIX] Fixed the wrongly dispose of GamePlayer.