Streaming Overlay: Add current leader's time or gap to leader

[MarkRoss-Eviden]

I'd be great for people watching the stream if they knew how close the current racer's time is to the current leader.

At events people can see the leaderboard, the track and the stream, if people are just watching the stream they need to look separate at the leaderboard to know what's going on. It'd be nice if by just looking at the stream the people watching and the current racer knew how they were doing relative to the leader.