Temporarily revert libglvnd support
    
    We had a mesa update go stable without a matching libglvnd update,
    leading to users getting partial mesa updates and making systems
    unbootable.
    
    This commit temporarily reverts the switch to libglvnd so that we could
    fix the situation in two steps:
    
    1) get a working mesa back to stable repos (addressed by this commit)
    2) push libglvnd using mesa + libglvn update out atomically
    
    https://pagure.io/fesco/issue/1675