[Mobile Replay]: Video clips are empty in the beginning

[c298lee]

On the brustolin org, most replays look like they're flickering when viewing them on Safari. This is because the video clips themselves are empty at the beginning. This gap at the beginning messes up some video players (browsers and desktop based players) including the video player used by Safari.

Example replay: https://brustolin.sentry.io/replays/c817d3d7daac4440a2788fd0cbf521b0
The third video clip in this replay and onwards contains around a second where the clip is just empty:

90_5579037_c817d3d7daac4440a2788fd0cbf521b0_2.video.mov

Note how Github doesn't let you play the empty first second of the clip. You'll be able to see the empty part if you download and play the clip using Quicktime Player.

We should ensure that we aren't creating videos with a gap at the start.

Relates to #82771