Paul Schrader had a special job on the set of his latest film, “Oh, Canada”: drawing on the jockstrap that Jacob Elordi wears in one of the Vietnam War drama’s pivotal scenes. There’s a choice at the heart of “Oh, Canada,” when the fictional filmmaker Leonard Fife (played as a young man by Elordi, and older man as Richard Gere) dodges the Vietnam draft and escapes to Canada. The script leaves breadcrumbs as to what exactly happens until very late in the film, but finally Elordi is seen reporting for an Army physical.