Shutdown shouldn't be repeated for stopped scenes

[samme]

If you do

this.scene.stop().stop().stop();

then SHUTDOWN is emitted three times.

This seems a little incorrect to me and it can also make shutdown event handlers needlessly complex.

I would rather have SceneManager#stop abort if the scene is already stopped.

[photonstorm]

Thanks for opening this issue, and for submitting a PR to fix it. We have merged your PR into the master branch and attributed the work to you in the Change Log. If you need to tweak the code for whatever reason please submit a new PR.