movenc: Write edit lists for fragmented files as well, if necessary

This is necessary to get the right timestamp offset for content
that starts with dts != 0.

This currently only helps when writing fragmented files with a non-empty
moov atom. When writing an empty moov atom, we don't have any packets
yet, so we don't know the starting dts for the tracks.

Signed-off-by: Martin Storsjö <martin@martin.st>