more refactoring, removed PhysicsInterface, cleaned up demos to make use of btDynamicsWorld derived classes.

removed two cached optimizations, type in btTransform and cached inverse transform (todo: test performance impact)
committed fixes that make the code adhere to 'who creates it, also destroys it'

src/BulletDynamics/Dynamics/btRigidBody.cpp

@@ -48,6 +48,9 @@ btRigidBody::btRigidBody( const btMassProps& massProps,btScalar linearDamping,bt
 
 	m_debugBodyId = uniqueId++;
 	
+	//m_internalOwner is to allow upcasting from collision object to rigid body
+	m_internalOwner = this;
+
 	setMassProps(massProps.m_mass, massProps.m_inertiaLocal);
     setDamping(linearDamping, angularDamping);
 	m_worldTransform.setIdentity();