MGM Resorts International is off to a flying start in 2013, filling all its hotel rooms for the International Consumer Electronics Show, anticipating a big burst of activity for the Year of the Snake and nailing down plans to put a half-billion dollars in improvements into its Las Vegas properties. And, on top of that, executives learned early Wednesday that they had cleared another hurdle in the process to build a second resort in the world’s most lucrative gaming market, Macau.