libwebp.go comes from gen2brain/webp v0.6.4 (libwebp v1.6.0 compiled to WebAssembly and translated to Go by wasm2go). It's copied with only its package name changed. encode.go is our own, written after that project's encoder.

It's copied rather than imported for two reasons:

the upstream package loads the system's libwebp with dlopen at startup whenever one is installed (unless every build passes -tags nodynamic);
it registers itself as the program-wide WebP decoder.

HotDog CMS decodes uploads with Go's own decoders and uses libwebp only to encode pixels those decoders produced.

Licenses: LICENSE (gen2brain/webp, MIT) and LICENSE.libwebp (libwebp, BSD-3-Clause).

To update: take libwebp.go from a newer gen2brain/webp release, change its first package webp line to package webpenc, check that Xencode keeps its signature (see lib/webp.c there), run go test ./internal/media/..., and update the versions above.