Having already grossed more than $10 million in previews, “ Backrooms ,” the latest feature from A24 and the debut directorial effort of 20-year-old Kane Parson, is already posed to be a box office hit.  Out in theaters nationwide now, the horror film, starring Chiwetel Ejiofor, Renate Reinsve, Mark Duplass, Finn Bennett, Avan Jogia, Lukita Maxwell and more, follows a therapist who must enter into the backrooms — a collection of liminal, eerie spaces originally inspired by 4chan memes — to find