Stretch for diff image

[astrofrog]

In some cases such as:

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

from astropy/astropy#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?