Mia Goth is terrific in “Pearl,” writer-director Ti West’s prequel to this year’s “X,” in which Goth played a dual role as the old lady killer who takes out a crew of young pornographers making a movie on her Texas ranch, as well as one of the young stars of the production within the film. “Pearl” is a lark — do we really need a separate film diving into the backstory of “X’s” killer? — but Goth, who is in nearly every frame of the film, is so deliriously transfixing that she makes the whole enterprise worthwhile. After “Pearl,” there’s nowhere Goth and West can’t go. It’s 1918 and Goth’s Pear…