DO NOT MERGE Update libpng to 1.6.20 am: 2642b28cda am: 17d9eb926b  -s ours
am: 6415f96a93  -s ours

* commit '6415f96a9305761d3cf85c09321e818ea49b64a4':
  DO NOT MERGE Update libpng to 1.6.20