added angular limits to the Generic D6 constraint. Works for small angles. Will add a check for different combinations, and use different extraction of ordering of rotation from the diff quaternion.

Improved vehicle interpolation of wheels, and added Z-up axis option for the Demo
made 'getWorldTransform' const method in btMotionState
added future 'deactivationCallback'(not used yet)

src/LinearMath/btDefaultMotionState.h

@@ -19,16 +19,25 @@ struct	btDefaultMotionState : public btMotionState
 	}
 
 	///synchronizes world transform from user to physics
-	virtual void	getWorldTransform(btTransform& centerOfMassWorldTrans )
+	virtual void	getWorldTransform(btTransform& centerOfMassWorldTrans ) const 
 	{
 			centerOfMassWorldTrans = 	m_centerOfMassOffset.inverse() * m_graphicsWorldTrans ;
 	}
 
 	///synchronizes world transform from physics to user
+	///Bullet only calls the update of worldtransform for active objects
 	virtual void	setWorldTransform(const btTransform& centerOfMassWorldTrans)
 	{
 			m_graphicsWorldTrans = centerOfMassWorldTrans * m_centerOfMassOffset ;
 	}
+
+	///Bullet gives a callback for objects that are about to be deactivated (put asleep)
+	/// You can intercept this callback for your own bookkeeping. 
+	///Also you can return false to disable deactivation for this object this frame.
+	virtual bool deactivationCallback(void*	userPointer) {
+		return true;
+	}
+
 };
 
 #endif //DEFAULT_MOTION_STATE_H