RENOWNED Bucks County portrait artist Nelson Shanks has painted everyone from Princess Diana to President Bill Clinton and from Pope John Paul II to Marisa Tomei. He and his wife, Leona, are the founders of Studio Incamminati, an art school on 12th Street near Callowhill. On a recent Friday, as the north light of a chilly afternoon flooded his Andalusia studio, Shanks, 77, chain smoked cigarettes and drank Diet Cokes while painting Gabrielle, a beautiful young belly dancer, on a 7-by-5-foot canvas. Following his 7-hour session with Gabrielle, Shanks spoke with Stephanie Farr about a hidden secret in his portrait...