Offenbach at Glyndebourne! Short of Die Soldaten with a picnic break or a period-instrument revival of Jerry Springer: The Opera, it’s hard to imagine a less probable operatic outcome— even this year. I mean, Offenbach: the saucy skewerer of middle-class pretension; the dazzling, vulgar arriviste of 19th-century opera. It couldn’t have been more incongruous had the sideburned showman himself razzed up, bass thumping, in a pimped Renault 5 and started pulling skids on the ha-ha.