George MacKay delivers a star-making performance in “1917.” After making his film debut at age 10 in the 2003 live-action “Peter Pan,” he played occasional film and TV roles and realized at age 19 “acting is what I wanted to do.” Since then, he’s appeared in an earlier WWI saga, “Private Peaceful,” and played Viggo Mortensen’s rebellious son in “Captain Fantastic” — and he has three more films coming up.