I'm angry at libwebp

[fwsGonzo]

With libavif, I can use the JPEG YUV planes directly without going through RGB when transcoding.

However, with libwebp the colors are not right.
The JPEG YUV decoding can be enabled here: https://github.com/varnish/tinykvm_examples/blob/main/cpp/webp/src/decode_jpeg.cpp
Using RGB the colors look correct, but it's probably not the fastest way to transcode JPEG to WebP