#75 Optipng fixes

[Samreay]

Fixes #75

Initially just updated optipng to take the path instead of a str, but then noticed a few odd things in the code that I changed as well

[smarie]

Same here - this is more readable of course, but maybe they had reasons to write the code that way in sphinx-gallery ?
Even if the original code is very strangely constructed IMHO, the less we modify it the more easily we can apply the same fixes than them.

Plus, since we did not port most of their tests yet in our code base (see #12), any modification is a risk of regression without CI safety harness.

[smarie]

Hi @Samreay , thanks a lot for this PR !
I made a few comments above.
I am planning to cut a release in the upcoming days, maybe this could ship in ?
Thanks again

[Samreay]

Let's see how this build goes

[smarie]

It seems that the fix is not currently tested in the CI :

WARNING - optipng binaries not found, PNG images and thumbnails will not be optimized

https://github.com/smarie/mkdocs-gallery/actions/runs/6928047749/job/18843066689?pr=77#step:6:125

Unfortunately optipng cannot be installed by pip. We therefore have to modify the github actions workflow to install it.
I will now merge this "as is" and open another issue where we'll have to find a way to include optipng in the CI.

Thanks again !