Log Message:
libwebp: update to 1.0.0.

- 4/2/2018: version 1.0.0
  This is a binary compatible release.
  * lossy encoder improvements to avoid chroma shifts in various circumstances
    (issues #308, #340)
  * big-endian fixes for decode, RGBA import and WebPPictureDistortion
  Tool updates:
    gifwebp, anim_diff - default duration behavior (<= 10ms) changed to match
                         web browsers, transcoding tools (issue #379)
    img2webp, webpmux - allow options to be passed in via a file (issue #355)