Changes made to pristine libwebp source by Moonchild Productions and mozilla.org developers.

2017/01/27  -- Synced with libwebp-0.6.0 (BZ #1294490).
2018/06/29  -- Synced with libwebp-1.0.0 + BUG=webp:381,383,384.
2019/01/21  -- Synced with libwebp-1.0.2