multimedia/ffmpeg: backport dav1d support

If both AOM and DAV1D are enabled FFmpeg defaults to libaom-av1,
so libdav1d needs to be selected explicitly. For example,

$ mpv --vd=libdav1d, --ytdl-format='bestvideo[vcodec=av01.0.05M.08]+bestaudio' \

'https://www.youtube.com/playlist?list=PLyqf6gJt7KuHBmeVzZteZUlNUQAVLwrZS'