“First of its kind in the region, the state-of-the-art display covering the western wall of the new WGBH headquarters debuted amid fanfare last year, when it began beaming an ever-changing array of images … to the half-a-million Boston-bound motorists who pass by it each week on the Massachusetts Turnpike.” But the screen, “marred by dark spots,” was turned off in June, and WGBH is suing the maker of its “digital mural.”