An intellectually stimulating art-house treasure all too easily overlooked amid the near-constant flood of Netflix content, “An Easy Girl” depicts a transformative summer in the life of a 16-year-old girl, but not the one described in the film’s title. That label — which writer-director Rebecca Zlotowski employs ironically, calling into question the patriarchal idea that a woman’s worth is tied up in how “hard to get” she plays it — refers to the protagonist’s 22-year-old cousin, no girl at all, but a comely temptress who breezes into the coastal French city of Cannes like a seductive tropical storm, turning heads and jostling perceptions wherever she goes.