Fix vehicles simulation instability when physics substepping is enabled

#2796

Source/Engine/Physics/PhysX/PhysicsBackendPhysX.cpp

@@ -1606,6 +1606,24 @@ PxMaterial* PhysicsBackendPhysX::GetDefaultMaterial()
     return DefaultMaterial;
 }
 
+void PhysicsBackendPhysX::SimulationStepDone(PxScene* scene, float dt)
+{
+#if WITH_VEHICLE
+    ScenePhysX* scenePhysX = nullptr;
+    for (auto e : Physics::Scenes)
+    {
+        if (((ScenePhysX*)e->GetPhysicsScene())->Scene == scene)
+        {
+            scenePhysX = (ScenePhysX*)e->GetPhysicsScene();
+            break;
+        }
+    }
+    if (!scenePhysX)
+        return;
+    scenePhysX->UpdateVehicles(dt);
+#endif
+}
+
 bool PhysicsBackend::Init()
 {
 #define CHECK_INIT(value, msg) if (!value) { LOG(Error, msg); return true; }
@@ -1895,10 +1913,6 @@ void PhysicsBackend::EndSimulateScene(void* scene)
         scenePhysX->Stepper.wait(scenePhysX->Scene);
     }
 
-#if WITH_VEHICLE
-    scenePhysX->UpdateVehicles(scenePhysX->LastDeltaTime);
-#endif
-
     {
         PROFILE_CPU_NAMED("Physics.FlushActiveTransforms");
 

Source/Engine/Physics/PhysX/PhysicsBackendPhysX.h

@@ -18,6 +18,7 @@ public:
     static PxCooking* GetCooking();
 #endif
     static PxMaterial* GetDefaultMaterial();
+    static void SimulationStepDone(PxScene* scene, float dt);
 };
 
 #endif

Source/Engine/Physics/PhysX/PhysicsStepperPhysX.cpp

@@ -3,7 +3,7 @@
 #if COMPILE_WITH_PHYSX
 
 #include "PhysicsStepperPhysX.h"
-#include "Engine/Physics/Physics.h"
+#include "PhysicsBackendPhysX.h"
 #include "Engine/Profiler/ProfilerCPU.h"
 #include <ThirdParty/PhysX/foundation/PxMath.h>
 #include <ThirdParty/PhysX/PxSceneLock.h>
@@ -66,7 +66,7 @@ void MultiThreadStepper::substepDone(StepperTask* ownerTask)
     }
 
     // -> OnSubstep
-    //Physics::OnSubstep();
+    PhysicsBackendPhysX::SimulationStepDone(mScene, mSubStepSize);
 
     if (mCurrentSubStep >= mNbSubSteps)
     {