Frame independent updates

[macskay]

At the moment the updates are frame-dependent rather than time-dependent. That should be updated

For this I suggest using composition. Have the Game class have a property called game_loop which is a callable. The callable is passed an instance of the game and is expected to implement the logic of one iteration of the main loop. Game will continuously call it so long as the stack is not empty.

This is essentially the strategy pattern. It will enable you to implement various mainloop strategies, such as FrameDependentMainLoop and TimeDependentMainLoop. It also makes it as easy as writing a single function for consumers of Taz to have their own completely custom game loop without having to subclass the Game class.