meta-surface-actor-x11: Fix unredirect heuristic

Damage coordinates are relative to the drawable not to the screen. So we
have to check whether x and y are 0 and not window_rect.x/y otherwise the
herustic will never trigger for windows on monitors whos x and y are not 0.

https://bugzilla.gnome.org/show_bug.cgi?id=738271

src/compositor/meta-surface-actor-x11.c

@@ -193,8 +193,8 @@ meta_surface_actor_x11_process_damage (MetaSurfaceActor *actor,
       MetaRectangle window_rect;
       meta_window_get_frame_rect (priv->window, &window_rect);
 
-      if (window_rect.x == x &&
-          window_rect.y == y &&
+      if (x == 0 &&
+          y == 0 &&
           window_rect.width == width &&
           window_rect.height == height)
         priv->full_damage_frames_count++;