glx: Implement GLX_EXT_no_config_context

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
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>