Fix route-path display of long legs.

Fix route-path display of long legs.

When leg spans more than a threshold (currently 0.5 degrees) of longitude, interpolate the actual path flown by the GPS/RM, which is a true great-circle. Previous we rendered a Rhumb line which does not agree at all. (Especially noticeable in the MapWidget and NavDisplay code, both of which use RoutePath to tesselate the route before rendering)