Apple softwares seem not to add a tref for the timecode (the next commit
fixes this issue), but at least FFmpeg does.

This can be used to generate a sample that demonstrates the feature:

    ./ffmpeg -f lavfi -i testsrc    \
             -f lavfi -i mptestsrc  \
             -f lavfi -i rgbtestsrc \
             -map 0 -map 1 -map 2   \
             -metadata:s:0 timecode=00:00:00:12 \
             -metadata:s:2 timecode=01:02:12:20 \
             -t 10 -y out.mov
    ./ffprobe out.mov

The timecode metadata being transmitted to the video streams, it can be
kept while transmuxed/transcoded.