(Reuters) – Visitors to the Museum of Modern Art in New York on Saturday had the chance to see an unusual performance: actress Tilda Swinton sleeping in a glass box. Photos of the "Burn After Reading" actress, posted on a web site called Gothamist, showed Swinton wearing shoes, a light blue shirt and dark blue pants, lying on white bedding with eyeglasses beside her. "Living artist, glass, steel, mattress, pillow, linen, water, and spectacles," said a description card of the performance called "The Maybe," according to a photo posted on Gothamist, a web site that covers New York. …