This gives track events to override default's we impose that are not always valid.

So, if a track event's setup changes these start or end, they are correct in popcorn, and the event fires them at the updated time, but our UI is off. In our case we needed this for sequencer, to enforce the time to be whatever was in the media's duration, but I abstracted it to allow any plaugin that may need to do this the ability to do it.