Also created a proper World struct in the process, to store all
infinite-extent type stuff.
Note that I goofed and did a new rustfmt pass but forgot to
commit before making these changes, so there's a lot of
formatting changes in this too. *sigh*
The part of the renderer responsible for light transport has been
split out into a LightPath struct. Also moving over to spectral
rendering, although it's a bit silly at the moment.
