Handle the possibility of losing lines in center line production.

[bbiscuit]

Right now we just ignore this as a possibility, but this is a very realistic scenario while we are driving along. We may lose both or (more likely) one of the lines. We need to at least have a special signal for when this happens, and not produce a centerline.

[bbiscuit]

This has been fixed over the course of just improving the centerline detection algorithm. If we lose the sideline, then we simply don't output a centerline, and we can react to that scenario.