Added renderObjects() to TiledMapRenderer

This is a very simple change. Instead of just calling `renderObject(object)` on the `TiledMapRenderer` on a loop for every layer, I created `renderObjects(layer)`, that contains the loop and calls `renderObject(object)`. This allows you to do any pre and post preparations before rendering the objects (like setting and restoring some values on the `spriteBatch`, my use case, but I'm sure I'll use it for more than that)

I also removed the extended `renderObject` on every subclass and added it only to the abstract base classes