This idea could be extended to other fields not presently considered to
be metadata, that would be handy to treat as such.

I use the key "id" because ffprobe outputs id= for streamid. Another
option could be to collect these types of metadata that go into
AVStream fields under a namespace like FFMPEG: or AVSTREAM: or
something, then delete all of them using AV_DICT_IGNORE_SUFFIX near the
end of of_open() since they're for internal ffmpeg use.

The FATE change is just because av_dict() changes the order of things
when elements are deleted.

/Tomas