Avoid use of buffers throughout thumbnailing

api/unstable/info.go

datastores/buffer.go

thumbnailing/i/01-factories.go

thumbnailing/i/apng.go

thumbnailing/i/flac.go

thumbnailing/i/gif.go

thumbnailing/i/heif.go

thumbnailing/i/jpegxl.go

thumbnailing/i/jpg.go

thumbnailing/i/mp3.go

thumbnailing/i/mp4.go

thumbnailing/i/ogg.go

thumbnailing/i/png.go

thumbnailing/i/svg.go

thumbnailing/i/wav.go

thumbnailing/i/webp.go

thumbnailing/thumbnail.go

thumbnailing/u/exif.go

thumbnailing/u/framing.go

thumbnailing/u/metadata.go