* Shell's output dips less than one percent * Exxon's production slides 7.5 percent By Andrew Callus and Anna Driver LONDON/HOUSTON, Nov 1 (Reuters) - The world's top oil and gas companies are struggling to deliver the output growth they need to outpace the burgeoning cost of exploration and development. Third-quarter results from Exxon Mobil, Royal Dutch Shell and other top international players released over the past few days mostly beat expectations thanks to a shortage of t