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.
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.
