Kevin Costner’s new Western epic “Horizon: An American Saga - Chapter 1” is the kind of film that begs the question, “how did this get made?” But we already know, because part of the film’s lore is that producer-director- star-co-writer Costner staked the funding himself, at great personal risk. In order to devote himself to the four-part “Horizon: An American Saga,” the Oscar winning director…