window.c: Don't allocate proxy manually

With the wayland change to automatically allocate the client side proxy
manually, we can now drop the code (and the FIXME) that did that and just
receive the proxy from the callback arguments.

clients/window.c

@@ -2226,7 +2226,8 @@ data_offer_destroy(struct data_offer *offer)
 
 static void
 data_device_data_offer(void *data,
-		       struct wl_data_device *data_device, uint32_t id)
+		       struct wl_data_device *data_device,
+		       struct wl_data_offer *_offer)
 {
 	struct data_offer *offer;
 
@@ -2235,12 +2236,7 @@ data_device_data_offer(void *data,
 	wl_array_init(&offer->types);
 	offer->refcount = 1;
 	offer->input = data;
-
+	offer->offer = _offer;
-	/* FIXME: Generate typesafe wrappers for this */
-	offer->offer = (struct wl_data_offer *)
-		wl_proxy_create_for_id((struct wl_proxy *) data_device,
-				       id, &wl_data_offer_interface);
-
 	wl_data_offer_add_listener(offer->offer,
 				   &data_offer_listener, offer);
 }