The clip sequencing test seems to have major time sync issues

[colinbdclark]

I can't quite explain what's changed, but it seems that the clip sequencer manual test is no longer working properly. It's worth investigating what has changed, and also consider the possibility of avoiding the use of Video.currentTime for synchronization, since it appears that it isn't frame accurate. Instead, we should synchronize a manual frame counter with the resolution of the video elements's play() and pause() promises.