glx: Nerf some GLX attributes we can safely ignore
If you try to point llvmpipe at a screen backed by the NVIDIA driver, you may be sorely disappointed, to the tune of "No matching fbConfigs or visuals found". The reason is that NVIDIA's driver supports mipmap generation for GLX_EXT_texture_from_pixmap, and 4 auxiliary buffers (!), and llvmpipe does not.
So, if we find those attributes set and the client-side driver claims not to support them, just erase their support from the GLX config list and carry on. Note that this code is generic for all DRI drivers, and that's fine. It would only matter if we'd implemented those features on the client side but hadn't yet restarted the X server to notice them, in which case you'd experience the same failure mode anyway.