Gus Van Sant is at his best when he points his camera at those on the outskirts of society, be they Good Will Hunting’s sharp-tongued janitor who happens to be a math genius, Drugstore Cowboy’s quartet of pharmacy-robbing addicts, or Milk’s gay politician fighting for LGBT rights from San Francisco City Hall. That’s precisely why the filmmaker is a perfect fit to tell the story of John Callahan, the quadriplegic Portland cartoonist whose work was marked by shaky lines and a giddy dismissal of tact. One famous Callahan comic features two klansmen in full regalia: “Don’t you love it,” one asks, “when they’re still warm from the dryer?”