Allocate client proxy automatically for new objects

When the server send a new object ID, the client used to have to allocate
the proxy manually and without type-safety.  We now allocate the proxy
in a client-side post-processing step on the incoming closure.

src/scanner.c

@@ -769,6 +769,8 @@ emit_structs(struct wl_list *message_list, struct interface *interface)
 
 			if (is_interface && a->type == OBJECT)
 				printf("struct wl_resource *");
+			else if (!is_interface && a->type == NEW_ID)
+				printf("struct %s *", a->interface_name);
 			else
 				emit_type(a);