clients: Add a widget_get_wl_subsurface

Adds a simple accessor for the wl_subsurface for widgets created with
window_add_subsurface.
diff --git a/clients/window.c b/clients/window.c
index 096e117..43761ca 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -1818,6 +1818,12 @@
 	return widget->surface->surface;
 }
 
+struct wl_subsurface *
+widget_get_wl_subsurface(struct widget *widget)
+{
+	return widget->surface->subsurface;
+}
+
 uint32_t
 widget_get_last_time(struct widget *widget)
 {