Also makes muxer shift the first video timestamp to zero, if it's not.
The trim position should respect the media timeline.
For example in a video that is 10s long (without edit list), if an edit list
adds 1_000ms to each video sample, and trimming 100ms, here's the expected:
- The video duration is 10.9s (`10s + 1s edit - 0.1s trim`)
- The first video frame time would be at 0.9s (`1s edit - 0.1s trim`)
PiperOrigin-RevId: 692187399
b0c6106882