Log Message:
libwebp: update to 1.3.1.

Upstream changes (from NEWS):

version 1.3.1
This is a binary compatible release.
* security fixes for lossless encoder (#603, chromium: #1420107, #1455619,
  CVE-2023-1999)
* improve error reporting through WebPPicture error codes
* fix upsampling for RGB565 and RGBA4444 in NEON builds
* img2webp: add -sharp_yuv & -near_lossless
* Windows builds:
  - fix compatibility with clang-cl (#607)
  - improve Arm64 performance with cl.exe
  - add Arm64EC support
* fix webp_js with emcc >= 3.1.27 (stack size change, #614)
* CMake fixes (#592, #610, #612)
* further updates to the container and lossless bitstream docs (#581, #611)

version 1.3.0
This is a binary compatible release.
* add libsharpyuv, which exposes -sharp_yuv/config.use_sharp_yuv
  functionality to other libraries; libwebp now depends on this library
* major updates to the container and lossless bitstream docs (#448, #546,
  #551)
* miscellaneous warning, bug & build fixes (#576, #583, #584)