backend/wayland: don't ack a configure event too early

Postpone sending xdg_surface.ack_configure until the guest compositor
commits a buffer, if at all.

Also see
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/issues/128.

backend/wayland/output.c

@@ -588,6 +588,11 @@ static bool output_commit(struct wlr_output *wlr_output,
 				output->surface);
 		}
 
+		if (output->has_configure_serial) {
+			xdg_surface_ack_configure(output->xdg_surface, output->configure_serial);
+			output->has_configure_serial = false;
+		}
+
 		wl_surface_commit(output->surface);
 
 		if (wp_feedback != NULL) {
@@ -742,7 +747,8 @@ static void xdg_surface_handle_configure(void *data,
 	assert(output && output->xdg_surface == xdg_surface);
 
 	output->configured = true;
-	xdg_surface_ack_configure(xdg_surface, serial);
+	output->has_configure_serial = true;
+	output->configure_serial = serial;
 
 	int32_t req_width = output->wlr_output.width;
 	int32_t req_height = output->wlr_output.height;

include/backend/wayland.h

@@ -95,7 +95,10 @@ struct wlr_wl_output {
 	// 0 if not requested
 	int32_t requested_width, requested_height;
 
+	uint32_t configure_serial;
+	bool has_configure_serial;
 	bool configured;
+
 	uint32_t enter_serial;
 
 	struct {