Feb 28 (Reuters) - Stock futures pointed to a lower opening for Canada's main stock index on Tuesday as commodity prices largely edged down ahead of a speech by U.S. President Donald Trump. Investors are shifting their attention to Trump's policy speech to a joint session of Congress on Tuesday night, when he is expected to provide clues on his economic plans. March futures on the S&P TSX index were down 0.14 percent at 7:15 a.m. ET. Canadian Producer Prices