Animation PlayMode.LOOP_RANDOM fixed

I noticed that in case `PlayMode.LOOP_RANDOM` is used in `Animation`, the `frameDuration` does not apply, but the texture region is changed in every frame.

This PR should fix that and keep the same texture region for as long as a frame duration. I used a small hack by using the last deltaTime to check which frameNumber there would have been in the past. I think that should be okay though.

It might happen though that because of the randomness the same texture region can be picked multiple times in a row which might not be what's expected. I'd expect it to choose a random region, but not the same one again. Because that would also contradict frameDuration somehow and makes the animation look... laggy.

But since I wasn't sure about that, I've kept it the way it was for now. Should that be changed as well?
This should not make the assumption that the animation was updated by `Gdx.graphics.getDeltaTime()`.