This patch adds the Ren'py Visual Novel engine split into four outputs:
- "out": contains the engine itself. Since Guix differs strongly from
the platforms Ren'py usually runs on, we ship our own Ren'py main file.
- "launcher": The Ren'py launcher, that people would normally expect
when invoking renpy -- it does not seem very usable in Guix, though.
- "the-question": An example game, that ships with Ren'py.
- "tutorial": The tutorial that ships with Ren'py.

Ren'py also ships its own version of pygame. They call it pygame_sdl2
since it's a reimplementation of pygame using SDL2, but the tags make
it very clear, that it is developed alongside Ren'py.