Find a way to automatize image conversion

Right now, both the .xcf and .png of the Verdigris logo are in the repository, but preferably, only the .xcf would be tracked and the .png built from it by CI. Unfortunately, the go-to tool xcftools cannot cope with the new .xcf version 11 which is used. imagemagick also doesn’t seem to work. gimpscript would probably be possible, but it is a huge amount of work.