Since “300” ended with the death of all major protagonists, a sequel makes about as much sense as a follow-up to “Titanic.” Nevertheless, Warner Brothers found a way to turn writer-director Zack Snyder’s enjoyable 2006 adventure film into an ongoing political saga punctuated by brutal, stylized killings and slow-motion geysers of bright, red blood. The movie was obviously designed as a cash grab, and the biggest surprise of the 103-minute feature is that it actually entertains.