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
am: 1c6b279894

* commit '1c6b2798941263e710c4053a1f5025f51730daf8':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: Ibaeac6c005f5c86a74cfe4d309afb70ef28e5428