hw/xwin/glx: publish GLX create_context extensions

Future work: To properly support GLX_ARB_create_context in indirect mode, we
need to use wglCreateContextAttribsARB() rather than wglCreateContext(),
when attribs are provided, rather than just dropping attribs on the floor,
as we currently do.

That probably entails removing the deferred context creation and instead
using a temporary window, as direct WGL does.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>