Improve the game loop

It can now skip frames. Replays are a *lot faster* on the "max speed" setting.
It was [after](https://github.com/OpenRA/OpenRA/blob/bleed/OpenRA.Game/Game.cs#L170) the logic and the rendering, that's why I moved it at the beginning of the logic, so that a) it runs after the rendering and b) it runs even if rendering is skipped.