qtdemux: Always use `tfdt` if available in BYTE segments
This reverts the decision from
https://bugzilla.gnome.org/show_bug.cgi?id=754230
where it was decided that we rather play safe and only use the tfdt if
it is "significantly different" to the sum of sample durations.
As the specification says
If the time expressed in the track fragment decode time (‘tfdt’) box
exceeds the sum of the durations of the samples in the preceding
movie and movie fragments, then the duration of the last sample
preceding this track fragment is extended such that the sum now
equals the time given in this box.we have to use the tfdt in general to allow for it to signal gaps in
the stream.
A muxer producing fragments might not yet know the full duration of the last sample of a previous fragment if the next fragment starts with a gap, and knowing the actual start of the next fragment would potentially require to violate latency requirements.
Additionally, the existence of tfdt allows to avoid accumulating
rounding errors from summing up the durations.
Or in short, @eocanha was absolutely right back then in the Bugzilla discussion :)