As imagined by British novelist P.D. James in “The Children of Men,” the very near future isn’t a place you’d ever want to visit. A worldwide infertility crisis threatens the human race, terrifying gangs prey upon the dwindling populace, and the desperate and elderly queue up for government-sponsored euthanasia. Yet as bleak as James’ vision might be, it can’t compare to the horrors dreamed up by filmmaker Alfonso Cuaron in adapting her novel for the screen. And that’s when Cuaron and his collaborators found that the more suffering they invented, the more credible they believed their movie became. “We didn’t want to do a science fiction movie,” says Cuaron, the director of “Y Tu Mama Tambien” and “Harry Potter and the Prisoner of Azkaban.” “We wanted to do a movie about the state of things.” John Horn reports.