glxgears: unbind current context before "destroying" it

glXDestroyContext does not destroy the context if it's still
connected to some window. Unbind context from window to test it.

Signed-off-by: Brian Paul <brianp@vmware.com>

progs/xdemos/glxgears.c

@@ -771,6 +771,7 @@ main(int argc, char *argv[])
    glDeleteLists(gear1, 1);
    glDeleteLists(gear2, 1);
    glDeleteLists(gear3, 1);
+   glXMakeCurrent(dpy, None, NULL);
    glXDestroyContext(dpy, ctx);
    XDestroyWindow(dpy, win);
    XCloseDisplay(dpy);