DO NOT MERGE Update libpng to 1.6.20 am: 0b39f202aa  -s ours am: 9795c8684c am: d0107515e0 am: 7d9fa2ce0f  -s ours am: fd386f8f05 am: 79bd206ce6
am: 557bd959eb

* commit '557bd959eb3daed875a58b4f9ac4ec2e90f3a67b':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: Ib832ed74d5856ac7748bb7cda2d0229f664bfb0b