Synchronize start of videos during recording export

[beasteers]

Now we generate videos by converting (and resampling to a constant fps) but different video streams may have different start timestamps which means that videos may be out of sync on the dashboard.

Where the conversion happens:
https://github.com/VIDA-NYU/ptgctl/blob/main/ptgctl/tools/local_record_convert2.py#L122-L141

The ways to fix this are either:

• find the maximum timestamp and drop frames across all streams before that timestamp
• find the minimum timestamp and replicate the start frame for those frames.

(do this for audio too, and end times as well?)