libwebp: upgrade 1.3.2 -> 1.4.0

libwebp: upgrade 1.3.2 -> 1.4.0

Changelog:
==========
* API changes:
  - libwebpmux: WebPAnimEncoderSetChunk, WebPAnimEncoderGetChunk,
                WebPAnimEncoderDeleteChunk
  - libsharpyuv: SharpYuvOptionsInit, SharpYuvConvertWithOptions
  - extras: SharpYuvEstimate420Risk
* further security related hardening in libwebp & examples
* some minor optimizations in the lossless encoder
* added WEBP_NODISCARD to report unused result warnings; enable with
  -DWEBP_ENABLE_NODISCARD=1
* improvements and corrections in webp-container-spec.txt and
  webp-lossless-bitstream-spec.txt
* miscellaneous warning, bug & build fixes

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>