https://bugs.freedesktop.org/show_bug.cgi?id=92059
--- Comment #14 from Ilia Mirkin imirkin@alum.mit.edu --- (In reply to Kai from comment #13)
So I'm not sure if the game actually *needs* the extension?
It does. Without the ext, the shader in question is invalid.
Mesa has partial support for the ext which gets enabled (along with every other available ext) by the force_glsl_bla thing, and that partial support is apparently sufficient for the needs of this game.