movenc: Remove a now redundant check

When using the new first_trun flag instead of checking the track id,
we don't need to have a special case for the separate_moof flag
any longer.

This simplifies the complicated codepath ever so slightly.

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Write correct presentation timestamps in tfra

Previously we wrote decoding timestamps here, while the specs
say it should be presentation timestamps.

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