Core/Movement: Use spline id generator for movement packets

src/server/game/Movement/Spline/MoveSplineInit.cpp

@@ -125,6 +125,7 @@ namespace Movement
 
     MoveSplineInit::MoveSplineInit(Unit* m) : unit(m)
     {
+        args.splineId = splineIdGen.NewId();
         // Elevators also use MOVEMENTFLAG_ONTRANSPORT but we do not keep track of their position changes
         args.TransformForTransport = unit->HasUnitMovementFlag(MOVEMENTFLAG_ONTRANSPORT) && unit->GetTransGUID();
         // mix existing state into new

src/server/game/Movement/Spline/MovementTypedefs.h

@@ -76,6 +76,7 @@ namespace Movement
 
     extern double gravity;
     extern float computeFallElevation(float t_passed, bool isSafeFall, float start_velocity);
+    extern UInt32Counter splineIdGen;
 }
 
 #endif // TRINITYSERVER_TYPEDEFS_H

src/server/game/Movement/Spline/MovementUtil.cpp

@@ -23,6 +23,7 @@
 namespace Movement
 {
     double gravity = 19.29110527038574;
+    UInt32Counter splineIdGen;
 
     /// Velocity bounds that makes fall speed limited
     float terminalVelocity = 60.148003f;