Automatic merge of T1.5.1-797-gabb8eb9a4 and 14 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 a055bca: 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 0a9d939: feat: Improved system information collection
- Pull request #899 at 28fe751: Duplex steam engines - Booster Engine addition
- Pull request #903 at 7353625: 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 #908 at 4b4afe3: feat: supports switching adhesion precisions
- Pull request #909 at c3a572a: Correct Control Car crash bug

Source/Orts.Simulation/Simulation/RollingStocks/MSTSControlTrailerCar.cs

@@ -41,7 +41,7 @@ namespace Orts.Simulation.RollingStocks
 {
     public class MSTSControlTrailerCar : MSTSLocomotive
     {
-        public int ControllerNumberOfGears = 1;
+        public int ControllerNumberOfGears = 0;
         bool HasGearController = false;
         bool ControlGearUp = false;
         bool ControlGearDown = false;
@@ -174,7 +174,7 @@ public override void Update(float elapsedClockSeconds)
                 {
                     var locog = car as MSTSDieselLocomotive;
 
-                    if (locog != null && car != this && !locog.IsLeadLocomotive() && (ControlGearDown || ControlGearUp))
+                    if (locog != null && locog.DieselEngines[0].GearBox != null && locog.DieselEngines[0].GearBox != null && car != this && !locog.IsLeadLocomotive() && (ControlGearDown || ControlGearUp))
                     {
                         if (ControlGearUp)
                         {
@@ -194,7 +194,7 @@ public override void Update(float elapsedClockSeconds)
                     }
 
                     // Read values for the HuD and other requirements, will be based upon the last motorcar
-                    if (locog != null)
+                    if (locog != null && locog.DieselEngines[0].GearBox != null && locog.DieselEngines[0].GearBox != null)
                     {
                         ControlGearIndex = locog.DieselEngines[0].GearBox.CurrentGearIndex;
                         ControlGearIndication = locog.DieselEngines[0].GearBox.GearIndication;