glx: Implement GLX_EXT_no_config_context

This is the GLX counterpart to EGL_KHR_no_config_context. Contexts may
now be created without reference to an fbconfig, in which case it is
treated as compatible with any fbconfig (and thus any GLX drawable).

Khronos: https://github.com/KhronosGroup/OpenGL-Registry/pull/102

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8999>