Build-time tool, which basically just rasterize SVG images (it doesn't
do anything special like gtk-encode-symbolic-svg which creates special
PNG for GTK to recolor them).
It looks like I had this prepared since 2018 according to file header,
but I just never finished doing it! :P
Basically now PNG icons of the Color icon themes do not need anymore to
be committed in the repository. They will be generated from the SVG
icons.
Also adding a missing icon from the 16px list (the Playground icon for
Preferences dialog was needed in 16x16 as well, yet missing).
Jehan