forward click events to layer surfaces

this was broken when we moved to swallowing mouse events that triggered "Frame"-context mouse bindings. layer surfaces don't run mouse binding logic, so they need mouse events unconditionally forwarded.

src/cursor.c

@@ -574,6 +574,8 @@ cursor_button(struct wl_listener *listener, void *data)
 		if (layer->current.keyboard_interactive) {
 			seat_set_focus_layer(&server->seat, layer);
 		}
+		wlr_seat_pointer_notify_button(seat->seat, event->time_msec,
+			event->button, event->state);
 		return;
 	}