Sometimes, video files will arrive with a timecode data stream
that causes `-map 0` to error out due to the stream not being
supported in the output container. These data streams generally do
not matter, so tell ffmpeg to ignore them rather than choking on
them.
With this change, the merge operator may join any number of media streams,
video or audio. The streams are downloaded in the order specified.
Also, fix the metadata post-processor so that it doesn't leave out
any streams.
