zopfli is a compression algorithm that optimizes the standard ZIP DEFLATE
compression. All of this is visually lossless, except for two minor things:
* lossy_transparent: remove colors behind alpha channel 0. No visual
difference, removes hidden information.
* lossy_8bit: convert 16-bit per channel image to 8-bit per channel.
This was done using the 'zopfli' package in Debian/buster, invoking this way:
zopflipng --iterations=50 --filters=01234mepb --lossy_8bit --lossy_transparent -y $f ${f}.zopfli.png
a6653300e8