In what can only be termed a devastating blow to an already-staggering industry, petroleum giant ExxonMobil has informed Boston public TV station WGBH that it will be terminating its support of PBS mainstay ‘Masterpiece Theatre’ after 2004. The company has bankrolled the program for more than three decades, and the pullout comes as PBS is discussing a controversial proposal to allow corporate sponsors to air 30-second ads on its supposedly commercial-free network of stations.