PRISMTeenage Dream did its job. With its five number one singles, the 2010 album turned Katy Perry into a genuine superstar, the kind of musician whose image rivaled her music in popularity, the kind who could topline her own 3 D theatrical documentary, the kind whose name became shorthand for a sugar pop sensibility. This meant there was only one thing left for her to do on its 2013 sequel, Prism: to make a graceful pivot from teen dream to serious,