The American frontier drama “The World to Come,” the Jim Broadbent and Helen Mirren comedy-drama “The Duke,” Gia Coppola’s “Mainstream,” and the Shia LaBeouf and Vanessa Kirby drama “Pieces of a Woman” are among the films set to premiere at the Venice International Film Festival in September, the first major event of its kind since the coronavirus pandemic forced the cancellation of large gatherings worldwide, organizers said Tuesday.