feat(track): 3D sleepers and switch timbers, blade heel and throw from templates, one rail carving pass

- sleepers3d / sleepers3dplain / sleepers3dbed in eu07.ini: timbers put together from the begin, main and end parts of a model
  and stretched to their length, plain sleepers of their own model; tracks whose plain sleepers would touch are laid as one
  cluster, lines of timbers led by priority and turned to the bisectors of frogs and crossings at a limited rate, whole timbers
  under crossings, split ones between tracks with room to part them, gaps filled per rail and overlaps shortened or dropped;
  clusters worked out in a background thread while driving, on demand in the editor
- the ballast can be left out under the beams (sleepers3dbed no)
- blades get their heel and throw from the switch template or blades <heel> <throw> in the scenery; blades of double switches
  run on into the track linking them to the next switch
- rail cutting at frogs, wing rails, movable noses and switch crossings done by one carving pass (RailCarve)
- tint of the rail treads applied to the colour only, without the matte band at the frog
- a switch trailed through with a movable nose set the other way derails the vehicle
- self-tests: sleepercheck, sharedzones, beamsnear, throw, sleepersbg

world/Segment.h

@@ -185,6 +185,9 @@ public:
 
     bool
         RenderLoft( gfx::vertex_array &Output, glm::dvec3 const &Origin, gfx::vertex_array const &ShapePoints, bool const Transition, double fTextureLength, double Texturescale = 1.0, int iSkip = 0, int iEnd = 0, std::pair<float, float> fOffsetX = {0.f, 0.f}, glm::vec3 **p = nullptr, bool bRender = true );
+    // a transition profile lofted in Steps pieces of equal length, so two paths of different make-up line up piece by piece
+    bool
+        RenderLoftSteps( gfx::vertex_array &Output, glm::dvec3 const &Origin, gfx::vertex_array const &ShapePoints, double fTextureLength, int const Steps ) const;
     // the profile lofted from distance From to To, moved outwards by Offsets and cut on its outer side to Extents beyond the gauge line, both as (distance, value) knots
     bool
         RenderLoftSpan( gfx::vertex_array &Output, glm::dvec3 const &Origin, gfx::vertex_array const &ShapePoints, loft_span const &Span ) const;