LWJGL2: Double Stopping the context is not supported

[MeFisto94]

Admittedly this is an artificial issue, but I will still open a PR and fix this.
Background: In my unit tests, I have a catch-all app.stop(true);. When the app has already been decreated once, this call creates a hangup in LwjglContext#waitFor (Line 442, createdLock.wait()), as noone will ever notify that lock anymore.

This fix is a simple if (needClose.get()) { return; }, however I need to give it a bit of thought, as needClose is also touched in case of uncaught exceptions. We'll see.

[MeFisto94]

A note here: uncaught exceptions call the listener which can decide whether the exception is "mission critical" or not, and thus just also sets needClose, in which case an app.stop() should also return instantly as the tear down is happening already and we can't judge whether it is past the part of the lock already.