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-drm.c b/src/compositor-drm.c
index 461fce7..b929728 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -2312,10 +2312,10 @@
 
 	destroy_sprites(d);
 
-	weston_compositor_shutdown(ec);
-
 	ec->renderer->destroy(ec);
 
+	weston_compositor_shutdown(ec);
+
 	if (d->gbm)
 		gbm_device_destroy(d->gbm);