OS ANGELES (Reuters) - It has wowed film festivals, won rave reviews and sparked Oscar buzz, but when "Brokeback Mountain," a.k.a. the gay cowboy movie, begins playing to general audiences on Friday, it faces its toughest challenge yet -- wooing mainstream America. [snip] ...Jake Gyllenhaal and Heath Ledger, risk alienating fans and sending their stars plummeting if, after watching the pair have sex on screen, audiences cannot see them any other way than gay. Winning over middle America is important for the roughly $12.5 million movie because its backer, art house specialist Focus Features, wants a big box office and...