Add support for viewport configuration

Current layer manager handles custom camera and render order.
Allowing to request a given viewport region would allow to simplify the creation of 2D / parallel scale overlays.

Viewport should probably be abstracted from VTK to provide:

- Fixed size
- Strech / dynamic size
- Anchoring system

Start by reading the current layer manager and how it handles custom cameras and render order. Define how a viewport abstraction should represent fixed and dynamic sizing plus anchoring without depending on VTK, then verify that 2D and parallel-scale overlays can request the required regions.

Written by the indexing model from the issue text.