One star. “The Great Wall” (103 minutes) PG-13. In Mandarin and English with subtitles. By Ann Hornaday, The Washington Post “The Great Wall” arrived last week with a lot at stake, being the most expensive co-production ever filmed in China, and the first one to feature a bona fide movie star, in the person of Matt Damon. The film met with early criticism for casting Damon as what many presumed would be the usual “white savior.” It turns out that his character, a mercenary soldier in an unspecified long-ago era who has come to China on the prowl for gunpowder, is less an out-and-out hero than a foil for ideas of national identity and cultural chauvinism that China is obviously eagerly to export for global consumption. To buy that message, though, viewers must first buy the “The Great Wall,” which, despite the hype that has surrounded it, feels like a letdown.