The sampling method used before is numerically unstable for very
small lights. That sampling method is still used for large/close
lights, since it works very well for that. But for small/distant
lights a simpler and numerically stable method is used.
It *seemed* to fix the problem I was running into, but it actually
made the SphereLight ray intersection code incorrect, and wa just
avoiding intersections that should have happened.
I should test better before committing. :-)
More specifically: prior to this, SurfaceLights returned the
shadow ray direction vector to use. That was fine, but it
kept the responsibility of generating proper offsets (to account
for floating point error) inside the lights.
Now the SurfaceLights return the world-space point on the light
to sample, along with its surface normal and error magnitude.
This allows the robust shadow ray generation code to be in one
place inside the renderer code.
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.
