Some older apps, e.g. games using version 3.0 or older of the glfw library try to use the old GLX_SGIX_fbconfig extension functions if present instead of the regular glXGetFBConfigs, glXGetFBConfigAttrib and glXCreateNewContext functions.
This commit adds implementations of these to libglvnd's libGLX fixing e.g. the "The Binding if Isaac: Rebirth" and "Crypt of the NecroDancer" games from Steam failing with a "GLX: Failed to retrieve visual for GLXFBConfig" error.
Note this commit does not add implementations for glXCreateGLXPixmapWithConfigSGIX and glXGetFBConfigFromVisualSGIX as these cannot be simply mapped to non extension functions and in practice these do not seem to be used.
Signed-off-by: Hans de Goede email@example.com