ci: Fix failing render-microfacet.opt test

[lgritz]

This test started failing sporadically recently, with output images that looked plausible, but differed slightly from the rest of the checked-in reference images for this test. It only failed maybe 5% of the time, so sometimes a CI run would pass, or sometimes fail just one job variation, or very occasional two. Rerunning the failed job would generally succeed, but sometimes fail again.

I'll cut to the chase: We were able to trace it to all failures happening on GitHub runners that identified themselves as "Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz". When subsequent runs of the same job variation succeded, it was always because it landed on a runner that identified itself as "AMD EPYC 7763 64-Core Processor".

We presume that some minor HW or SW differences (perhaps in a library with an ISA-specific code path) resulted in LSB errors that conspire to sample different light paths on a minority of pixels, leading to a different pattern of specular "fireflies".

The expedient fix herein is simply to commit yet another reference image that is considered to be a "pass".

Along for the ride, discovered during my debugging:

• Rename the label of one job variant, which was incorrect.
• Add comments to the test thresholding parameters (comments ported from the analogous script in OIIO, making it easier to understand next time one of us is figuring out this code).