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/TrackRails.h

@@ -15,14 +15,30 @@ http://mozilla.org/MPL/2.0/.
 namespace track_rails {
 
 void forget();
+// changes with every forget(): what's kept of the rails around holds as long as it doesn't
+unsigned int memo_stamp();
 rail_size measure_rail( gfx::vertex_array const &Profile );
+// the noses, wing rails and point rails at crossings at small angles are carved out of what lies around them, all by the same rules: no rail
+// reaches into the flangeway of another one where its wheels run, and two heads lying over each other share the room between them by what
+// the wheels of each roll on
+void carve_plans( TTrack &Track, rail_layout &Layout );
+// the layout of the track designed already to carve the tracks around against, before carving
+rail_layout const *designed_layout( TTrack &Track );
+// how many parts carving left with nothing of the head somewhere along them, since the start
+int carve_faults();
 // how far a wing rail reaches past the crossing point of the rails
 double wing_reach( double const Slope );
 double check_length( double const Slope );
+// how far from the tip the nose of a movable frog bends over to the wing rail
+double movable_nose_length( double const Slope );
 // the rails of the track are cut where they cross other rails: plain tracks, and switches whose frog is made from the rails
 bool cut_rails( TTrack const &Track );
 int blade_steps( TTrack const &Track, int const Path );
 double blade_length( TTrack const &Track, int const Path );
+// the plain track a path of a switch goes on into at its end, laid the same way, which the blade of the path may run on into
+TTrack const *runon_track( TTrack const &Switch, int const Path );
+// how far the blade of the switch the track goes on from runs on into its rail of Side
+double blade_runon( TTrack const &Track, int const Side );
 material_handle cut_material( material_handle const Material );
 float tread_brightness( material_handle const Material, glm::vec2 const &Band );
 glm::vec3 tread_colour( material_handle const Material, glm::vec2 const &Band );
@@ -33,6 +49,16 @@ gfx::userdata_array worn_userdata( std::size_t const Count );
 // the ends of rails left open: at track ends with nothing joined, blade tips, the gap of a frog
 void cut_faces( gfx::vertex_array const &Strip, std::size_t const Ring, bool const Start, bool const End, gfx::geometrybank_handle const &Bank, TTrack::geometryhandle_sequence &Geometry );
 
+// the geometry of a track laid while one is alive goes into the chunks of the pool given, of the same type, before new ones
+class reused_chunks {
+public:
+    reused_chunks( chunk_pool *Pool );
+    ~reused_chunks();
+private:
+    chunk_pool *m_previous { nullptr };
+};
+gfx::geometry_handle insert_chunk( gfx::vertex_array &Vertices, gfx::userdata_array &Userdata, gfx::geometrybank_handle const &Bank, int const Type );
+
 TSegment const &path_segment( TTrack const &Track, int const Path );
 int path_count( TTrack const &Track );
 TTrack *path_neighbour( TTrack const &Track, int const Path, int const End );