Ah, to de-glam. It’s one of the surest shortcuts to newfound artistic appreciation: a bedraggled deviation into dowdy drama by a beautiful star. Acclaim by way of sweatpants. “Cake,” in which Jennifer Aniston plays a bitterly grieving, caustically acerbic and chronically pained Los Angeles woman, belongs to a contrived kind of low-budget movie — drab and depressed, but predictably poignant — just as artificial as any blockbuster convention. As Claire Simmons, Aniston has facial scars, stringy hair and a slightly frumpier frame. But this is also a very recognizable Aniston, whose deserved appeal has always depended on marrying her pert all-American girl-next-door with a glib sarcasm. In “Cake,” she has turned up her cynicism nob as far as it will go.