update: strip all metadata from PNGs

This strips metadata and optimizes the compression of all PNGs copied
from the app's source repo as well as all the icons extracted from the
APKs.  There have been exploits delivered via image metadata, and
F-Droid isn't using it all, so its best to just remove it.

This unfortunately uncompresses and recompresses the files.  Luckily,
that's a lossless procedure with PNGs, and we might end up with
smaller files.  The only tool I could find that strips without
changing the image data is exiftool, but that is written in Perl.