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/RailProfile.cpp

@@ -63,6 +63,14 @@ rail_size measure_rail( gfx::vertex_array const &Profile ) {
         }
     }
     result.tread = result.head;
+    auto flat { -1e9 };
+    for( std::size_t i = 0; i < count; ++i ) {
+        auto const &point { Profile[ i ].position };
+        if( point.y > top - 0.0005 ) {
+            flat = std::max( flat, static_cast<double>( point.x ) );
+        }
+    }
+    result.shoulder = std::clamp( result.head - flat, 0.0, 0.5 * result.head );
     std::vector<std::size_t> heads, lips;
     for( std::size_t i = 0; i < count; ++i ) {
         auto const &point { Profile[ i ].position };