Imagine, for a moment, that when Steve McQueen rode his motorcycle up barbed wire in The Great Escape, it was the first time that American movie audiences had been exposed to the concept of Nazi prison camps. That will give you some idea of the unusual challenge facing Peter Weir’s The Way Back. This is a prison-break movie that becomes an epic travelogue: A motley group of men slip through barbed wire in Siberia and set out on an astonishing trek southward toward freedom, across Russia, Mongolia, and Tibet, and finally into India. But it’s also the first major motion …