use position_ms to determine timestamps

[goatzillax]

Description of Problem & Solution Hi, smee again!

I have some pretty unclean video files due to network/terrible hardware issues which have a lot, lot, lot of dropped frames, which totally wreaks havoc on timestamps and the "copy" output. Preprocessing the files through ffmpeg was unsavory as well as... didn't totally fix the situation.

Opencv appears to have position_ms which seems to fix this. As a test I plumbed in a position_ms to VideoJoiner to pluck it from the VideoStreamCv2 member, and that seems to work. It produces correct timestamps that ffmpeg is able to correctly use to copy clips with.

TBH I don't know how this would interact with the other options, but for my single-file case it seems to work so far.