add improved btGeneric6DofSpring2Constraint, thanks to Puhr Gabor and Tamas Umenhoffer!

improved the new demo testbed (work-in-progress)
add basic Lua demo, import URDF test, STL import, obj import

src/BulletCollision/CollisionDispatch/btCollisionObject.cpp

@@ -35,6 +35,7 @@ btCollisionObject::btCollisionObject()
 		m_restitution(btScalar(0.)),
 		m_internalType(CO_COLLISION_OBJECT),
 		m_userObjectPointer(0),
+        m_userIndex(-1),
 		m_hitFraction(btScalar(1.)),
 		m_ccdSweptSphereRadius(btScalar(0.)),
 		m_ccdMotionThreshold(btScalar(0.)),

src/BulletCollision/CollisionDispatch/btCollisionObject.h

@@ -92,11 +92,10 @@ protected:
 	int				m_internalType;
 
 	///users can point to their objects, m_userPointer is not used by Bullet, see setUserPointer/getUserPointer
-	union
-	{
-		void*			m_userObjectPointer;
-		int	m_userIndex;
-	};
+
+    void*			m_userObjectPointer;
+    
+    int	m_userIndex;
 
 	///time of impact calculation
 	btScalar		m_hitFraction; 

src/BulletCollision/CollisionDispatch/btCompoundCollisionAlgorithm.cpp

@@ -232,7 +232,9 @@ void btCompoundCollisionAlgorithm::processCollision (const btCollisionObjectWrap
 		m_compoundShapeRevision = compoundShape->getUpdateRevision();
 	}
 
-
+    if (m_childCollisionAlgorithms.size()==0)
+        return;
+    
 	const btDbvt* tree = compoundShape->getDynamicAabbTree();
 	//use a dynamic aabb tree to cull potential child-overlaps
 	btCompoundLeafCallback  callback(colObjWrap,otherObjWrap,m_dispatcher,dispatchInfo,resultOut,&m_childCollisionAlgorithms[0],m_sharedManifold);