Changes made to pristine libwebp source by mozilla.org developers.
2018/01/21 -- Synced with libwebp-1.0.2 (bug 1521478).
2018/11/26 -- Synced with libwebp-1.0.1 (bug 1509878).
2018/10/04 -- Synced with libwebp-1.0.0 (bug #1294490).