Zero day images have spiky noise

[mmp]

There's a lot of spiky noise in the zero day scenes that looks like an MIS issue:

Things learned so far:

• It even happens with --force-diffuse, so it's presumably not an issue with concentrated indirect light
• It happens with both the path and volpath integrators (and both the CPU and GPU volpath integrators), so it's presumably not an issue with MIS computations in the integrators
• It is being added when a BSDF-sampled ray hits a light source (hence, the assumption that something is wrong with MIS weights)
• It happens with both solid angle sampling and uniform area sampling of the triangular light sources, so is not related to e.g. solid angle sampling blowing up for very small triangles.
• It happens with both the BVH and the "power" light sampler, so is presumably not related to the BVH light sampler e.g. incorrectly returning zero probability for sampling a light.
• It doesn't happen with "maxdepth" 1.

...and that's as much as I've figured out so far.

[mmp]

And also: it still happens if bump mapping and per-vertex normals are disabled, so is presumably not a shading normals issue..

[mmp]

Many things have been fixed since this and the noise now seems reasonable to the point that it now seems plausibly expected noise from tricky indirect lighting.