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 am: cc5a2eaea1
am: b11b5e94a3  -s ours

* commit 'b11b5e94a35dc25b2962b0288b75b6a9ea2e52b8':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I524229ce18a5ceaebfeefe52a6986afe05c78738