Preparing for integration into nom::VisualUnitTest ...

* Add SDL2's rendering driver selection (index) onto
nom::RenderWindow's ::create method -- this is for ease of access with
libRocket.

* Use nom::RenderWindow exclusively within nom::RocketSDL2Renderer
interface and declare public.

* Introduce nom::RocketSDL2Renderer::gl_init convenience method.

* Introduce nom::RocketSDL2SystemInterface::ActivateKeyboard &&
nom::RocketSDL2SystemInterface::DeactivateKeyboard methods -- for SDL2
unicode text input bridging with libRocket.

* Add event polling for SDL_TEXTINPUT; this enables text input on form
elements with libRocket.

* Use nom::Event && nom::EventHandler in libRocket's event loop.
Fix early termination of loop in RocketSDL2Renderer