Update CI to minify PNGs

[aneshodza]

As seen in this CI run I had the issue that GitHub would just leave bigger images blank. That would look as follows:

Minifying the images fixed that issue, so I thought maybe we add it to the CI. For the moment its on level two, but it goes all the way from zero to seven.
My 2nd idea was, that the default should be level zero and then adding a notice in the readme, that minifying images may be necessary if there are blank images like that one.

[openscript]

Thank you for taking the time and contributing to this project! It's interesting, that the image was just not rendered because it's big. That shouldn't happen and I will investigate it.

As a user I expect that everything is rendered in the quality I supply the artifacts. Microsoft Word or Adobe product also don't compress documents without the users consent. So I think a better enhancement would be, to create another compressed version. I think we should compress the whole document instead of only PNGs.

Compressing PDFs can be easily achieved with ghostscript:

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile=output.pdf input.pdf

[aneshodza]

I'm wondering if that would have actually fixed the issue. I'll try taking the version with the blank images and running gs on it before downloading.

I expect that everything is rendered in the quality I supply the artifacts

I agree. That's why I suggested setting the compression level to 0 and adding a notice in the readme, that blank images may be fixed by changing the compression level.

[aneshodza]

I wasn't able to run gs for some reason, so I couldn't investigate that further. What I did instead is add a notice that compression is possible in the README and turned it off by default.

[aneshodza]

I investigated a bit further into optipng, the tool I'm using for the compression. They mention that "OptiPNG is a PNG optimizer that recompresses image files to a smaller size, without losing any information." on their website, meaning that the only downside to using optipng would be a longer CI run. They are using rfc1951, which is based on lz77 and huffman, both lossless compressions.

I expect that everything is rendered in the quality I supply the artifacts

This is still the case, even after running optipng.

Due to making the CI (maybe unnecessarily) longer, I think the correct approach would still be to deactivate it per default, but add a notice in the readme.