I forgot to mention that I reported this against Mesa 12.0.3 and LLVM 3.9.0, but the situation was the same when I played the game earlier with Mesa 11.x. I also tried 13.0-dev, but aside from some increase in performance (the worst part is when defending the wall in Vergen at the end of chapter 2), I saw no change in visuals. In the game's launcher there is an option to select between OpenGL 2.1 and 3.2, but I see absolutely no difference between them.