Melling’s part in “Pillion” — an imaginative role that leaves little to the imagination — solidifies a career shift since his “Potter” days that has taken him from tragic (a golden-voiced actor without arms or legs who is exploited in “The Ballad of Buster Scruggs”) to cunning (the prince Malcolm in the Denzel Washington-led adaptation of Shakespeare’s “Macbeth”) to charming (a young Edgar Allan Poe in “The Pale Blue Eye”) to risqué (“Pillion”).