+ add option to set pre-tick callback, called at the beginning of each internal simulation step

+ use real-time for soft body demo (using this pre-tick callback)
+ optimize the generation of bending constraints for the special case where the distance is 2

src/BulletDynamics/Dynamics/btDiscreteDynamicsWorld.cpp

@@ -362,6 +362,10 @@ void	btDiscreteDynamicsWorld::internalSingleStepSimulation(btScalar timeStep)
 	
 	BT_PROFILE("internalSingleStepSimulation");
 
+	if(0 != m_internalPreTickCallback) {
+		(*m_internalPreTickCallback)(this, timeStep);
+	}	
+
 	///apply gravity, predict motion
 	predictUnconstraintMotion(timeStep);