At 36, he is already one of the art world’s brightest lights, painter of portraits that borrow heavily from the old to make something blazingly new. Where once there were only white kings and their queens, Kehinde Wiley inserts the “brown faces” long absent from Western art. Rappers, athletes, kids off the street. Wyatt Mason hangs with Wiley as he hits the beaches and markets of North Africa, handpicks his subjects, and transforms them, step by inspired step, into an ambitious new series of paintings. This is how a masterpiece is made