Use wl_resource_get_user_data for weston_surface resources

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/text-backend.c b/src/text-backend.c
index f940bbf..27afdff 100644
--- a/src/text-backend.c
+++ b/src/text-backend.c
@@ -188,7 +188,7 @@
 	wl_list_insert(&text_input->input_methods, &input_method->link);
 	input_method_init_seat(weston_seat);
 
-	text_input->surface = surface->data;
+	text_input->surface = wl_resource_get_user_data(surface);
 
 	input_method_context_create(text_input, input_method);