x/image/webp: issue with colors contrast when converting to jpeg/png

[badoux]

When i try to convert a webp image to a PNG image doing like that: https://play.golang.org/p/yAGNTfQrx2

The converted PNG dont keep the same "contrast" as the source webp image, while it's ok when converting the webp image with:
dwebp in.webp -o out.png

The same thing occurs when i convert a WEBP to JPEG.

• webp image (in.webp) : https://files.fm/f/dygrmekf
• png image (converted with dwebp cmd) : https://files.fm/f/hx2thfdn
• png image (converted with webp.Decode()->png.Encode()) : https://files.fm/f/2hws9psu

Im using go1.8 darwin/amd64

[octopepper]

Works well using C libwebp with https://github.com/chai2010/webp