Seven years after the release of her 2013 debut feature “Palo Alto,” which was nominated for the Horizons Award at the Venice Film Festival, Gia Coppola is back with her sophomore drama, “Mainstream.” The film, which centers on a trio of young, fledgling Angelenos—Maya Hawke, Andrew Garfield and Nat Wolff—who rise to Internet fame, world premieres Saturday as part of Venice’s Horizons section.