movenc: Add a flag for indicating a discontinuous fragment

This allows creating a later mp4 fragment without sequentially
writing the earlier ones before.

Normally when writing a fragmented mp4 file sequentially, the
first timestamps of a fragment are adjusted to match the
end of the previous fragment, to make sure the timestamp is the
same, even if it is calculated as the sum of previous fragment
durations.

When writing an individual mp4 fragment discontinuously like this
(with potentially writing the earlier fragments separately later),
there's a risk of getting a gap in the timeline if the duration
field of the last packet in the previous fragment doesn't match up
with the start time of the next fragment.

Using this requires setting -avoid_negative_ts make_non_negative
(or -avoid_negative_ts 0).
movenc: Expose the fragment index as an avoption

This allows setting the right fragment number if doing
random-access writing of fragments.