Remove dependency on xcftools

[silveira]

The script https://github.com/silveira/openpixels/blob/master/extract_layers.sh currently depends on xcftools (xcfinfo and xcf2png).

xcftools is not longer easily available (no longer on Debian https://tracker.debian.org/pkg/xcftools), it has had vulnerabilities (j-jorge/xcftools#12).

An alternative could be rewriting that script in Python with the gimpformats library.