Gia Coppola’s first feature Palo Alto chronicled teenagers stumbling toward adulthood way back in distant 2013; her new Mainstream, bowing in Venice’s Horizons section, features a trio of 20-somethings plundering the Internet culture of their time, bartering their values for big cash and followers on social media but still, of course, looking for love. It's a messy, childish scrawl of a film, but it is high on energy.