Any discussion of Richard Linklater’s “Boyhood” necessarily begins with how the film was made, because that alone is practically a miracle. Linklater began filming in 2002. He cast 6-year-old Ellar Coltrane as Mason, the boy of the title, his own 8-year-old daughter, Lorelei, as Mason’s sister Samantha, and Patricia Arquette and Ethan Hawke as their divorced parents.