With the previous patches applied, we now have crash due to use-after-free when
a client exits without deleting all it's GL contexts

On client exit, CloseDownClient first calls glxClientCallback() with
ClientStateGone, which calls __glXFreeContext() directly.

Subsequently CloseDownClient() frees all the clients resources, which leads to
ContextGone() being called for a context resource where the context has already
been freed.

Fix this by modifiying glxClientCallback() to free the context resource.

Also make __glXFreeContext() static, as calling it directly leads to this
problem, instead the context resource should be released.

With the previous patches applied, this can be demonstrated with e.g. glxinfo,
which doesn't delete it's context before exit.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Adam Jackson <ajax@redhat.com>