S.T.A.L.K.E.R. Clear Sky, the game prequel to S.T.A.L.K.E.R. Shadow of Chernobyl (FPS PC game we appreciated a lot last year here at Roumazeilles.net) is now available for download at the Steam video game distribution web site of Valve. More, it will only be available there (no boxed DVD on the shelves). It seems that this…