LONDON, May 16 (Reuters) - European stock index futures pointed to a sharply lower opening on Wednesday, with major bourses set to plumb fresh 2012 troughs as investors fret about a possible Greek exit from the euro zone and falling global demand for commodities. At 0625 GMT, futures for Euro STOXX 50 Germany's DAX, France's CAC and Britain's FTSE 100 were all down between 0.9 and 1.1 percent. The euro hit a four-month low versus the dollar in Asian trade as investors continued to dit