Replaced GafferScene::Render base class with RendererAlgo.h header.

The base class didn't provide any useful functionality as far as abstraction was concerned - it was just a place where we put utility functions the derived classes needed for talking to renderers. Since that functionality could be useful elsewhere, it makes more sense to provide it in a namespace available to all.

There is similar functionality buried in SceneProcedural that we should move to RendererAlgo.h at a later date. For now we're just moving the code from the Render class, because we need to move the Render class out of the way to do implement some needed refactoring of the Executable code.
WIP