Implement fog in raytracer

[kpreid]

In principle, this is simple: we already have support for volumetric rendering, and we just need to count empty space as containing the fog. In practice,

• the current code has lots of assumptions that empty space can be skipped and has zero effect other than possibly being the end of a semi-transparent material, and
• the fog we are trying to render has a non-uniform density, so we need a special calculation for it.