Willie Osterweil: Modern filmmakers who want to accurately convey the evils of slavery could do so through the stories of Toussaint Louverture or Nat Turner, Harriet Tubman or Frederick Douglass — or any of the other thousands of slaves who didn’t look to white saviors to escape their bondage. But you’d never know from watching Hollywood movies that a single slave ever freed herself. “Django Unchained,” “Glory,” “Lincoln” — these films all feature the benevolent intervention of white protagonists. Even Steven Spielberg’s “Amistad,” about a slave uprising, is as much about the white lawyers arguing the slaves’ case. A particular narrative about slavery is told over and over: The institution was a historical aberration perpetrated by evil white people, but luckily there were good white people who listened to the black people, and they helped free the slaves, and now it’s all over.