Introduce explicit base render backend type

The main idea behind the render backend is to decouple low level bits
from scenes. The end goal is to make the render backend provide render
targets where the scene can render.

Design-wise, such a split is more flexible than the current state, for
example we could start experimenting with using qtquick (assuming that
the legacy scene is properly encapsulated) or creating multiple scenes,
for example for each output layer, etc.

So far, the RenderBackend class only contains one getter, more stuff will
be moved from the Scene as it makes sense.