Render properly in LevelScreen and InfiniteScrollingLevelScreen

Also fixes the problem of update() being called twice in LevelScreen
Revert renderTop method
Extract methods for rendering different parts

Default behavior is provided in LevelScreen, but subclasses may override
each render method if they have special behavior
Rework LevelChunk so it can render entities and backgrounds seperately
InfiniteScrollingScreen fits with new LevelScreen rendering methods
Merge pull request #32 from SweatyReptile/rendering

Render properly in LevelScreen and InfiniteScrollingLevelScreen