Peter Debruge Chief Film CriticAs statues to Confederate heroes are torn down around the country, the question of whom to honor in their place poses an intriguing challenge — one that writer-director Mark Amin seems to have anticipated with his abolitionist adventure movie “Emperor.” Essentially a filmic monument to a scarcely documented American hero, “Emperor” tells the virtually unknown story of Shields Green, a descendant of African royalty who was born into slavery and later escaped,