Stretch for diff image

In some cases such as:

https://output.circle-artifacts.com/output/job/65b009f4-252a-490f-a3da-61cb064aa4e0/artifacts/0/results/fig_comparison.html

from https://github.com/astropy/astropy/issues/15029 - the diff doesn't show any changes, but downloading it and stretching it shows a couple of pixels different. I wonder if we could stretch the PNGs to maximize the use of the available dynamic range to make it easier to see what is going on in subtle cases like this?

Start by reproducing the linked CircleCI diff from the astropy/astropy issue, then trace pytest-mpl's image-comparison path to find where the PNG diff is generated. Done means subtle pixel differences are visible through the generated diff image without changing the comparison result; the issue names no specific file or test.

Written by the indexing model from the issue text.