Includes:
- More scene parsing code. Making good progress!
- Making the rendering code actually use the Scene and Assembly
types.
- Bare beginnings of a Tracer type.
Weird, to be frank. It was a lot of work. Can't believe I don't
even remember doing it before. Oh well.
In any case, I've improved the 'old' one quite a bit. It should
be more robust now, and will provide errors that may actually be
useful to people when a file fails to parse.
