Use oxipng instead optipng

[K0-RR]

Oxipng began as a complete rewrite of the OptiPNG project, which was assumed to be dead as no commit had been made to it since March 2014.
The core goal of rewriting OptiPNG was to implement multithreading, which would be very difficult to do within the existing C codebase of OptiPNG. This also served as an opportunity to choose a more modern, safer language (Rust).

[douglasjunior]

Good to know, thanks!