Add automatic frameskip support

[JD557]

I think it should be easy to add a frameskip definition to the RenderLoop. If some frame takes too long to render, the RenderLoop just has to switch the canvas to a FrameSkipCanvas that ignores any rendering operations (clear(Backbuffer), putPixel,redraw, ...) and applies all other operations on the real canvas.

This might need some smart logic to avoid constant frameskipping (maybe setting a maxFrameskip instead of frameskip).