Halfway through “Pen15″ comes a scene we’ve seen a million times before. A teenager, fumbling to understand their own changing body, figures out that masturbating can feel amazing, only to have their well-meaning mother open the door and catch them in the act. This pubescent horror story is so ubiquitous onscreen that it’s practically become a requisite in most any coming of age story, but there’s one basic, crucial difference between that cliché and “Pen15’s” take. That humiliated teen is almost always a boy. “Pen15,” by deliberate contrast, hands that scene over to a girl, who...