DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007 am: a0b5142c6c am: 34dcc2af83  -s ours
am: 3aec76d873  -s ours

* commit '3aec76d8738b35055700dfe9f6b7d5d0cb758de4':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: Ic12af96bba174c6dc036c4ceb796a7a9966c96c2