GLOBAL MARKETS-Stocks, dollar dip as Fed policy-makers meet

* Federal Reserve starts meeting, eyes on possible tapering signal * U.S., European shares back off Monday's gains, bonds steady * Euro gains vs dollar after boost on upbeat German data * Bond yields slip; gold, oil fall after recent push By Richard Leong NEW YORK, Dec 17 (Reuters) - Financial markets were cautious on Tuesday as The U.S. Federal Reserve began a two-day policy meeting from which investors await possible signals on when it plans to start winding down its mass