PNG difftool/mergetool for git?

Not sure if anyone still reads this list, but...

It would be useful if PNG had a chunk type for difftools/mergetools to
use, in particular git difftools/mergetools, so that PNGs can be checked
into git and later diffed and merged. In particular, in case of merge
conflicts, a PNG should be able to contain all the conflicting data. Oh
yeah, the tool would also be responsible for identifying what counts as
a merge conflict, so some sort of configuration would be useful.
(perhaps in-band, with another chunk? seems unnecessarily complicated
tho, out-of-band would be more flexible and more useful.)