View chunk-level diffs of PNG files

1. Download zlib and build it with ./configure and make.
2. Download pngcheck.
3. Make sure that the zlib directory is called zlib and is a sibling of of the pngcheck directory.
4. Build pngcheck with make -f Makefile.unx.
5. Put pngcheck somewhere in your path.
6. Create a pngdiff shell script somewhere in your path:

   #!/bin/bash
   
   LOCAL_FILE=`mktemp /tmp/pngdiff.local.XXXXXX` || exit 1
   pngcheck -v $1 > $LOCAL_FILE
   
   REMOTE_FILE=`mktemp /tmp/pngdiff.remote.XXXXXX` || exit 1
   pngcheck -v $2 > $REMOTE_FILE
   
   bbdiff --wait --resume $LOCAL_FILE $REMOTE_FILE

   (replacing bbdiff with your preferred text diffing tool)
7. Add the following lines to ~/.gitconfig:

   [difftool "pngdiff"]
     cmd = pngdiff "$REMOTE" "$LOCAL"

8. Run git difftool -t pngdiff to view diffs.

Relatedly, to view diffs of images in Preview.app (using the up/down arrows to toggle between the old and new versions), add this to ~/.gitconfig:

[difftool "imagediff"]
  cmd = open -n -W -a preview "$REMOTE" "$LOCAL"