Ensure that the video is recording for the whole duration of the experiment

For now, there is a lag between the start of the experiment and the start of the video recording, probably due to a lag at the start of the FFmpeg process.

In order to fix the problem, we will need two things:

1. Find a way to track the output FFmpeg process that does the camera capture. There are some possible solution in this ffmpeg-python issue.
2. Find a way to make the two threads communicate (for instance, via queues).