Ginger & Rosa, the latest and possibly most accessible picture to come from British filmmaker Sally Potter, represents a coming of age for Elle Fanning as much as it does for the character she plays. Structurally, the film is pretty standard stuff; as the Ginger of the title, Fanning confronts or falls into situations beyond her age bracket and goes through the painful emotional transformation from child to woman in a scant eighty four minutes of narrative. But Potter has never been a standard director, nor should Fanning be seen any longer as a standard actress. Amazingly, Ginger & Rosa proves an astronomical leap forward for the latter and a…