A flood of natural gas swamping the U.S. is turning into a global glut, sinking prices and dimming the hopes of American producers to export their way out of an oversupplied domestic market. Natural-gas futures have fallen 25% over the past 2½ months. The declines continued Tuesday, with April futures dropping 3.45% to $2.938 a million British thermal units on the New York Mercantile Exchange. Shares of gas-production companies are…