Comment 18 for bug 932125

The problem is that you don't have libglcore.so as part of proprietary drivers (nvidia, for e.g.). At this time libglcore is substituted with mesa-dri/glx (which is abs. soft-ware rasterizer, not using your video card 3D acceleration) and xserver-xorg-video-nouveau (queer soft-ware subst. for native video card drivers).

Try to run glxgears, glxinfo and you'll see error - glxinfo: error while loading shared libraries: libGLcore.so.1: cannot open shared object file: No such file or directory