DO NOT MERGE Update libpng to 1.6.20 am: cf8a579007 am: a0b5142c6c am: 34dcc2af83  -s ours am: 3aec76d873  -s ours am: 9cac6eee10 am: c34faaeec7
am: 8ecc2c0512  -s ours

* commit '8ecc2c05123c5774012e423ed139d18fa41ad058':
  DO NOT MERGE Update libpng to 1.6.20

Change-Id: I4cdbd424cbd8cb381a4736b6dcb3915f2c2633c9