movenc: Write a 0 duration in mdhd and tkhd for an empty initial moov

ISO/IEC 14496-12:2012/Cor 1:2013 is explicit about how this should be
handled. All zeros doesn't mean that the full file has got a zero
duration, only that the track samples described within the initial moov
have got zero duration. An all ones duration means an indeterminate
duration.

Keep writing a duration consisting of all ones for the ISM mode -
older windows media player versions won't play a file if this is
zero. (Newer windows media player versions play either version fine.)

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Don't write any tfdt atom for ismv files

The tfdt atom shouldn't be needed in those cases, we already
write tfxd atoms for ismv anyway, which is roughly equivalent.

This avoids having to declare the iso6 brand for ismv files.

Signed-off-by: Martin Storsjö <martin@martin.st>
movenc: Don't write any iso brands in ismv files

We deviate slightly from the iso specs for these files.

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