Calling `set_higher_ts` before offsets are known pollutes
`highest_video_ts` with timestamps that are out of range of actual
timestamps. They're generally somewhere high above 0 until an
offset for all streams is found, where they are then reset to 0 or
slightly below 0 in the presence of b-frames.
`highest_video_ts` also needs to start below 0 for the same reason.
Even with timestamps being reset to close to 0, b-frames will
cause initial DTS to drop below 0, thus we need a value that should
"always" be below any "real" timestamps observed.
`highest_video_ts` tracking now only starts once all input streams
are ready, and is computed based on all buffered packets at that
point.
Ruwen Hahn
d966742a63