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
am: 9dfb357e98  -s ours

* commit '9dfb357e98730c0505cf0df4f0842e8fafcf181a':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I1472b4d493015e5eb8c8b9f11cb106eb1de93384