movenc: copyts with use_tfdt does not copy base media decode time

[thinkski]

When transcoding an MP4 segment to another MP4 segment, it is useful to be able to use the same base media decode time within the tfdt tag, so that segments can be transcoded in parallel.

Specifying copyts and use_tfdt should achieve this, but does not, instead producing an output MP4 where the base media decode time begins with 0.

How to reproduce:

% ffmpeg -copyts -i in.mp4 -movflags use_tfdt -f mp4 -movflags cmaf+delay_moov+skip_trailer out.mp4

% mp4dump in.mp4
...
    [tfdt] size=12+8, version=1
      base media decode time = 151213553719664
...

% mp4dump out.mp4
...
    [tfdt] size=12+8, version=1
      base media decode time = 0
...