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

* commit '79bd206ce6c5266b1ca00a84641020d7c902eb31':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I87f93ced4442df0c7b9b02ade74b07e4e8469c4b