Reorganized light and surface traits so that light sources are
surfaces as well, which will let them slide easily into
intersection tests with the rest of the scene geometry.
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 lighting is super crappy, and pretty much hacked in. Will
need to redo this properly soon. However, this verifies that
certain other parts of the code are (mostly) working properly.
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.
