Various shell scripts improvements, switch to oxipng

[C0rn3j]

Conversation in #471

If I get the time I might fix it further as current inkscape version does not carry -e and -z is deprecated.

If someone wants to beat me to it, see https://invent.kde.org/plasma/breeze/-/merge_requests/437 for inspiration, it will also cut down the runtime when it's batched.

[baxysquare]

I really like the idea of improving the shell scripts for the repositiory. But I'm not a programmer and don't have the means to review the scripts to see how they would affect the build process for the main project. We could really use some help from @LibretroAdmin or another top contributor, if at all possible.

[RobLoach]

I too am a fan of script updates here. My issue is that while it seems like oxipng is a better option, it isn't highly adopted. Not available in Debian repos, for instance.

[C0rn3j]

Whatever version Debian would be shipping would likely be old enough as to be undesirable.

oxipng has .deb releases available on their GitHub, however - https://github.com/shssoichiro/oxipng/releases/latest - should be fine to use those