Button stays "pressed" when clicked "rapidly" in non-continuous rendering mode...
Calling Gdx.graphics.requestRendering() in a ClickListener doesn't solve the problem.
One might change the `visualPressedDuration` like this :

```
// hach for releasing the click in non continous rendering
ClickListener.visualPressedDuration = 0f;
```

but it's pain in the ass when switching from continuous to non continous rendering.