Move webp pixbuf loader to platform

This needs 3 👍 from release team members.

webp is everywhere now, right-click an image to save in your browser, chances are it'll be a webp

Our platform and sdk should support webp in pixbufs so that apps can stop adding the loader themselves or fail to support a widespread image format.

See

• https://github.com/search?q=org%3Aflathub%20webp-pixbuf-loader&type=code
• https://github.com/workbenchdev/Workbench/pull/837

Untested, not sure how I can test this as org.gnome.Platform without waiting for nightly.