“Exit Thru the Gift Shop,” the documentary starring iconic street artist Banksy, seems like another one of the British prankster’s elaborate hoaxes — not that anyone will ever get the chance to ask him. “He’s the most brilliant artist you’ll never get to meet,” “Super Size Me” director Morgan Spurlock joked to guests at a screening he hosted at the Tribeca Grand. Later, at an after-party at Meet on Crosby Street, MySpace president Jason Hirschorn mingled with Nobu’s Drew Nieporent, John Turturro, Moby, Michael Stipe and David Cross, star of the up coming BBC series “The In creasingly Poor Decisions of Todd Margaret.”