pixman-renderer, gl-renderer: Destroy debug bindings on clean up

Also make sure backends destroy the renderers before shutting down the
compositor to avoid a double call to weston_binding_destroy().

This is a step towards making renderers switchable during runtime.
diff --git a/src/compositor-x11.c b/src/compositor-x11.c
index d99bde5..ee3df31 100644
--- a/src/compositor-x11.c
+++ b/src/compositor-x11.c
@@ -1443,10 +1443,10 @@
 	wl_event_source_remove(compositor->xcb_source);
 	x11_input_destroy(compositor);
 
-	weston_compositor_shutdown(ec); /* destroys outputs, too */
-
 	ec->renderer->destroy(ec);
 
+	weston_compositor_shutdown(ec); /* destroys outputs, too */
+
 	XCloseDisplay(compositor->dpy);
 	free(ec);
 }