What a time it is to be a pop star. When the arena tours have done a year of sales, you can share the experience with fans who couldn’t score a ticket via the magic of the multiplex. Taylor did it with Eras, Beyonce did it with Renaissance, immortalizing their mega-shows and collecting some extra cash. Between concert films and jukebox biopics, movies that feature familiar music have become one of the surer things in a shaky industry.  Now, curiously enough, it’s Billie Eilish’s turn and she’s doing so by sharing co-directing credit with none other than James Cameron and in 3D, no less. The experiment and its result is mildly perplexing.