Dennis Hanthorn is Atlanta Opera’s new general director. But even before he officially begins, he’s had to make some difficult decisions. “With an almost 50 percent drop in subscriptions to date, Hanthorn eliminated a quarter of the opera’s performances for the coming year to avoid going deeper into debt. Four productions are still on the 2004-05 calendar: “Carmen,” “Don Giovanni,” “La Bohème” and “Fidelio.” But the total number of performances will be trimmed from 16 to 12.”