If there's a keynote performance at the 11th annual Tribeca Film Festival, it may well be Abbie Cornish's riveting portrayal of a Texas single mother who, desperate for money to regain custody of her son, haphazardly smuggles Mexican immigrants across the border. Such leading roles don't frequently come around for women, but this year's Tribeca boasts a boatload of them. In David Riker's "The Girl," which will make its world premiere in competition at the festival, Cornish's fraught, sweaty performance...