This adds `libpng-apng'. I had this in the dependency graph of palemoon,
a webbrowser I stopped to work on because of license/trademark/legal
insecurities.

libpng-apng can be used to unbundle it from icecat. Despite the comment
in gnu/packages/gnuzilla.scm: This is a real, maintained software which
bases on top of libpng and tracks its releases.

When libpng is updated, this package needs to be updated aswell as soon
as the updated release of the patch is out.

I have appended the last version I packaged as a git format-patch file.

In the description the 2 occurences of APGN (Animated Portable Network Graphics)
could be written with the @defn{} of texinfo I think,
and the synopsis could be changed to just "Animated Portable Network
Graphics library". I think back then I just copied upstrema in text.

Should anyone be interested to finish the work on the webbrowser, the
repository with the branch is available at request.