backend/wayland: wait for xdg_surface.configure explicitly

We were assuming a roundtrip was enough to get an
xdg_surface.configure event. That's not the case, the protocol
spec doesn't make such a guarantee.

backend/wayland/output.c

@@ -718,6 +718,7 @@ static void xdg_surface_handle_configure(void *data,
 	struct wlr_wl_output *output = data;
 	assert(output && output->xdg_surface == xdg_surface);
 
+	output->configured = true;
 	xdg_surface_ack_configure(xdg_surface, serial);
 
 	// nothing else?
@@ -868,7 +869,14 @@ struct wlr_output *wlr_wl_output_create(struct wlr_backend *wlr_backend) {
 			&xdg_toplevel_listener, output);
 	wl_surface_commit(output->surface);
 
-	wl_display_roundtrip(output->backend->remote_display);
+	struct wl_event_loop *event_loop = wl_display_get_event_loop(backend->local_display);
+	while (!output->configured) {
+		int ret = wl_event_loop_dispatch(event_loop, -1);
+		if (ret < 0) {
+			wlr_log(WLR_ERROR, "wl_event_loop_dispatch() failed");
+			goto error;
+		}
+	}
 
 	output_start(output);
 

include/backend/wayland.h

@@ -91,6 +91,7 @@ struct wlr_wl_output {
 	struct zxdg_toplevel_decoration_v1 *zxdg_toplevel_decoration_v1;
 	struct wl_list presentation_feedbacks;
 
+	bool configured;
 	uint32_t enter_serial;
 
 	struct {