This is the way the world ends, or starts to end, in George Clooney’s “The Monuments Men”: with a bang. In the film’s opening scenes, Hubert and Jan van Eyck’s “Ghent Altarpiece” is loudly dismantled, panel by panel, and prepared—too late, though—for a secret hiding place. The clock is ticking for its fellow masterpieces. World War II is raging and Hitler, an unpromising art student before he became Der Führer, fancies himself