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/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);