In keeping with the current cinematic age, even a slasher/porn-centric franchise gets a “universe” these days, as “MaXXXine” caps off an unlikely trilogy from writer-director Ti West and star Mia Goth. Stepping up in class in terms of casting, the film works best as an ode to the movies and grime of 1980s Hollywood, and a bit less as a mystery built around its driven starlet-in-waiting.