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 test scene isn't rendering properly, presumably because
something isn't correct in the parsing (although it's not clear
it's in the mesh parsing). Need to investigate.
