To make a convincing zombie, it’s all in the pacing. George Romero invented the modern zombie with his 1968 film “Night of the Living Dead,” and he still likes them old-fashioned — slow-moving but hard to stop. “I don’t know if you’re aware of the rift in the zombie community,” says Ruben Fleischer, director “Zombieland.” He’s all for speeding up the undead.