“Horizon: An American Saga — Chapter 1” marks Kevin Costner’s return to the Western genre that brung ‘im into stardom nearly 40 years ago. This first part constitutes a stern handshake agreement with his audience: Just get through these first three hours, people. Quit your bellyaching. You know what these folks had to do to survive? Things could perk up and get rolling come August, when Chapter 2 of this reverently labeled “American Saga” continues in theaters, to be followed by Chapter 3 (currently filming) and then, finances and distribution/streaming arrangements with Warner Bros. and Max w…