Automatic merge of T1.5.1-870-ge0bf062eb and 16 pull requests

- Pull request #570 at 3539862: Experimental glTF 2.0 support with PBR lighting
- Pull request #839 at d00beb9: First phase of https://blueprints.launchpad.net/or/+spec/additional-cruise-control-parameters
- Pull request #876 at f92de76: docs: add source for documents previously on website to source Documentation folder
- Pull request #882 at 9c456aa: Blueprint/train car operations UI window
- Pull request #885 at 8f94333: feat: Add notifications to Menu
- Pull request #886 at 6c0785b: Scene viewer extension to TrackViewer
- Pull request #892 at 1f5ba4c: Signal Function OPP_SIG_ID_TRAINPATH
- Pull request #896 at 5866028: First implementation of https://blueprints.launchpad.net/or/+spec/specific-sounds-for-ai-trains
- Pull request #897 at 42f1dd9: feat: Improved system information collection
- Pull request #903 at 9bead33: Downloading route content (Github, zip)
- Pull request #907 at 9b0b04f: Bug fix for https://bugs.launchpad.net/or/+bug/2047300 Dynamic tracks disappear after long tunnel
- Pull request #911 at 6834af0: docs: Add refactoring as a special type of PR
- Pull request #912 at d595703: New Triple Valve Features Vol. 2
- Pull request #914 at e06fbf1: Adjustments to Duplex steam
- Pull request #915 at 6d911d7: Correct calculation error with curve friction
- Pull request #917 at 3bbf66e: Lighting Configuration Enhancements

Author: openrails-bot

Source/RunActivity/Viewer3D/Lights.cs

@@ -270,6 +270,10 @@ public static void CalculateLightCone(LightState lightState, out Vector3 positio
 
         bool UpdateState()
         {
+            // No need to update lights if there are none
+            if (Car.Lights == null)
+                return false;
+
 			Debug.Assert(Viewer.PlayerTrain.LeadLocomotive == Viewer.PlayerLocomotive ||Viewer.PlayerTrain.TrainType == Train.TRAINTYPE.AI_PLAYERHOSTING ||
                 Viewer.PlayerTrain.TrainType == Train.TRAINTYPE.REMOTE || Viewer.PlayerTrain.TrainType == Train.TRAINTYPE.STATIC, "PlayerTrain.LeadLocomotive must be PlayerLocomotive.");
 			var leadLocomotiveCar = Car.Train?.LeadLocomotive;

Source/RunActivity/Viewer3D/Trains.cs

@@ -102,8 +102,7 @@ internal void Mark()
             foreach (var car in cars.Values)
             {
                 car.Mark();
-                if (car.lightDrawer != null)
-                    car.lightDrawer.Mark();
+                car.lightDrawer?.Mark();
             }
             CABTextureManager.Mark(Viewer);
         }
@@ -151,8 +150,7 @@ public void PrepareFrame(RenderFrame frame, ElapsedTime elapsedTime)
                 car.PrepareFrame(frame, elapsedTime);
             // Do the lights separately for proper alpha sorting
             foreach (var car in cars.Values)
-                if (car.lightDrawer != null)
-                    car.lightDrawer.PrepareFrame(frame, elapsedTime);
+                car.lightDrawer?.PrepareFrame(frame, elapsedTime);
         }
 
         TrainCarViewer LoadCar(TrainCar car)
@@ -165,7 +163,8 @@ TrainCarViewer LoadCar(TrainCar car)
                 car is MSTSLocomotive ? new MSTSLocomotiveViewer(Viewer, car as MSTSLocomotive) :
                 car is MSTSWagon ? new MSTSWagonViewer(Viewer, car as MSTSWagon) :
                 null;
-            carViewer.lightDrawer = new LightViewer(Viewer, car, carViewer);
+            if (car.Lights != null) // Don't make a light viewer when there are no lights
+                carViewer.lightDrawer = new LightViewer(Viewer, car, carViewer);
             return carViewer;
         }
     }