GLOBAL MARKETS-Dollar up, stocks pare gains as central banks meet

* Dollar index recovers from five-week low * U.S. stock indexes surrender gains; agricultural shares down * Aussie dollar skids after RBA chief hints at rate cut * Chinese central bank injects cash for first time since February NEW YORK, July 30 (Reuters) - Stock indexes surrendered gains on world markets on Tuesday as investors positioned ahead of this week's meetings of the U.S., British and euro zone central banks, while the dollar climbed from its recent five-week low.