window: Send ack_configure immediately from configure handler

Once we've updated the window state and scheduled a resize, we know that
the next frame we send to the compositor will match the configured state.
This means we can just ack the configure immediately and not jump
through hoops to try to do it from the redraw stage.

clients/window.c

@@ -230,8 +230,6 @@ struct window {
 	int fullscreen;
 	int maximized;
 
-	int next_attach_serial;
-
 	enum preferred_format preferred_format;
 
 	window_key_handler_t key_handler;
@@ -1341,12 +1339,6 @@ surface_flush(struct surface *surface)
 		surface->input_region = NULL;
 	}
 
-	if (surface->window->next_attach_serial > 0) {
-		xdg_surface_ack_configure(surface->window->xdg_surface,
-					  surface->window->next_attach_serial);
-		surface->window->next_attach_serial = 0;
-	}
-
 	surface->toysurface->swap(surface->toysurface,
 				  surface->buffer_transform, surface->buffer_scale,
 				  &surface->server_allocation);
@@ -3893,7 +3885,7 @@ handle_surface_configure(void *data, struct xdg_surface *xdg_surface,
 				       window->saved_allocation.height);
 	}
 
-	window->next_attach_serial = serial;
+	xdg_surface_ack_configure(window->xdg_surface, serial);
 
 	if (window->state_changed_handler)
 		window->state_changed_handler(window, window->user_data);