glx: Avoid atof() when computing the server's GLX version

glx: Avoid atof() when computing the server's GLX version

atof() is locale-dependent (sigh), which means 1.3 becomes 1.0 if the
locale's decimal separator isn't a full-stop. Just use the protocol
major/minor instead. This would be slightly broken if the server
generically implements 1.3+ but a particular screen is only capable of
less, but in practice no such servers exist.

Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/74
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>