Image: A24 Liminal horror movie Backrooms is set to become A24's biggest hit, with $10.4 million clipping through in preview screenings, earning back its production budget, and signaling a $40-50 million opening weekend. Alex Garland's "Civil War," the company's previous top earner, made $2.9 million in previews. Adapted by Kane Parsons from his own low-budget, high-concept online series, the film draws from message-board lore an endless maze of faded, yellowing offices, a world inspired by strip malls, business parks, hotel basements and other liminal spaces of suburban America. Parsons' web series documented a fictional research group investigating these eerie environments; in the movie, stars Chiwetel Ejiofor, Renate Reinsve and Mark Duplass are hapless everday explorers. It's been getting good reviews, too, holding an 89% score on Rotten Tomatoes. The location depicted in the photo that started it all has been found ; it had been f ound all along.…