I now get thumbnails for MKVs:
{F6588567}
Supposedly it is auto-detected but documentation says
> either `avcodec_register()` or `avcodec_register_all()` must be called before any other libavcodec functions.
`av_register_all()` internally calls `avcodec_register_all()`. It also makes sure it is only initialized once, not everytime this is called.
Perhaps someone more knowledgable in ffmpeg can chime in?