Director-writer Martin Hynes shapes his first movie into something emotionally truthful, painfully funny and vibrantly alive. It’s a near-perfect road movie, since you don’t want the ride to end. Oregon teenager Mercer White, played by a subtly sensational Lou Taylor Pucci, hits the highway in a stolen car to track down his half-brother, Arlen (Jsu Garcia), who doesn’t know their mother has died. In concise episodes that brim over with the pleasures of the unexpected, Hynes deepens Mercer’s…