fix(track): what the review of unevenness found in the engine

- A path of no length gave a joint with a vector across of NaN to the path
  joined to it, on any scenery, with unevenness or not. It now has no
  vector across, and so makes no joint.
- Axles no longer stray along the track. It moved vehicles against each
  other, changing the distance their couplers go by.
- The bend under an axle goes straight on past the ends of a path: the
  polynomial ran off on a short curved one, with a kick in the cab shake.
- A track left out of its rail run, where tracks are joined one way only,
  no longer walks the whole run again on every call.
- The clatter of an axle goes by the outer axle on its side of the vehicle,
  and not always by the same one, as the two can be on different tracks.
- A vertex works out the amounts once, and the foot of the ballast not at all.

world/Segment.h

@@ -13,6 +13,7 @@ http://mozilla.org/MPL/2.0/.
 
 #include "utilities/Classes.h"
 #include "rendering/geometrybank.h"
+#include "world/TrackUneven.h"
 #include <glm/glm.hpp>
 
 struct map_colored_paths {
@@ -77,7 +78,9 @@ class TSegment
     // moves a lofted vertex with the unevenness of the track
     void settle_vertex( gfx::basic_vertex &Vertex, glm::dvec3 const &Origin, glm::vec3 const &Position, glm::vec3 const &Parallel, double const Distance ) const;
     // displacement of a point by the unevenness, at a distance along the path
-    glm::dvec3 stray( glm::dvec3 const &Point, double const Distance ) const;
+    glm::dvec3 stray( glm::dvec3 const &Point, track_uneven::amounts const &Amount, double const Distance ) const;
+    // a point of the path at a distance, past its ends straight on: the polynomial would run off on a short curved path
+    glm::dvec3 point_extended( double const Distance ) const;
     void design_position( double const fDistance, glm::dvec3 &position, glm::vec3 &rotation ) const;
 
 public: