In Due Date, a skinny, scowly and dryly self-referential Robert Downey Jr. meets a chubby, beardy, quasi-autistic Zach Galifianakis boarding a flight from Atlanta to Los Angeles. Downey Jr. plays Peter, a Bluetoothed architect with a very pregnant wife (Michelle Monaghan) waiting at home for him; Galifianakis’s Ethan is a…