Introducing RenderGroupPipeline

[bitbrain]

Introduce a new RenderPipeline which handles rendering in a different way than the previous ones (layered).

The Problem

At the moment rendering works fine, it is possible to stack layers onto each other and apply shaders to them. It is also possible to re-order these layers to have different rendering results.
However, this does not help when wanting multiple Viewport per layer. For example, we want to have a "world layer", which has a different world zoom than the UI layer. Furthermore, the world layer itself might not be on one layer at all. Multiple game objects might get rendered on different layers, depending on their type/group (see #96).

The Proposal

Introduce a new "grouped" pipeline which is more dynamic than the already existing one. The new default pipeline should fulfill these requirements:

• by default, the pipeline should consist of a single pipe which should reflect the default behavior of libgdx
• each layer has a getViewport() and setViewport methods to retrieve/set the current viewport for that particular layer
• each layer allows tweening such as alpha transitions

< WIP >