NOW we know what the Lau brothers do with those massive million-dollar directors' fees they pay themselves . . . they buy art. At least one of them, any way. Mr Joseph Lau Luen Hung, is regarded as one of the world's leading collectors of Chinese art. According to a special report on Arts and Antiques in yesterday's International Herald Tribune, Mr Lau ''only goes for the best - and takes good advice''. Not only does he take their advice, he also pays top dollars for his purchases and, more often than not, sets world record prices. Last year he set a record when he paid US$2.86 million (HK$22.3 million) for a 16th Century porcelain jar.