libwebp: Update to 1.6.0

[akien-mga]

- 6/30/2025 version 1.6.0
  This is a binary compatible release.
  API changes:
    - libwebp: WebPValidateDecoderConfig
  * additional x86 (AVX2, SSE2), general optimizations and compression
    improvements for lossless
  * `-mt` returns same results as single-threaded lossless (regressed in
    1.5.0, #426506716)
  * miscellaneous warning, bug & build fixes (#393104377, #397130631,
    #398288323, #398066379, #427503509)
  Tool updates:
    * cwebp can restrict the use of `-resize` with `-resize_mode` (#405437935)

[Repiteo]

Thanks!