[test-depth] Use a gint for width, not guint, when calculating -width/2

It was a fluke that this worked out due to how clutter_actor_set_depth
internally converts the incorrect integer result to fixed point.

tests/interactive/test-depth.c

@@ -26,7 +26,7 @@ raise_top (gpointer ignored)
 static ClutterActor *
 clone_box (ClutterTexture *original)
 {
-  guint width, height;
+  gint width, height;
   ClutterActor *group;
   ClutterActor *clone;