Improved SteerForSphericalObstacles

- Previous version was using the distance to the obstacle's center, not
  to the intersection point. This means that further away but larger
  obstacles, where the intersection was closer, weren't being given as
  high an avoidance as the closer but much smaller obstacles where the
  actual intersection was further away in time.
- Avoidance strength no longer depends on the detected obstacle count but
  on how far we're from a likely intersection.
- While we still take into account the obstacles we won't collide with,
  we give them much lower avoidance priority.