BulletSim: add VEHICLE_ more parameter value limit checking.

This only bounds passed parameters as there is no good way of refusing
the parameter setting. This mostly means that passing NaN's won't
crash the simulator.

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -5216,6 +5216,10 @@ namespace OpenSim.Region.Framework.Scenes
             return null;
         }
 
+        // Get terrain height at the specified <x,y> location.
+        // Presumes the underlying implementation is a heightmap which is a 1m grid.
+        // Finds heightmap grid points before and after the point and
+        //    does a linear approximation of the height at this intermediate point.
         public float GetGroundHeight(float x, float y)
         {
             if (x < 0)