movenc: Write tfdt atoms

These allow getting the absolute start timestamp of a fragment
without reading preceding timestamps. This fixes sync between
tracks if starting from fragments in different streams that don't
align exactly.

This also is a prerequisite for producing DASH content.

Signed-off-by: Martin Storsjö <martin@martin.st>
mov: Handle tfdt atoms

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