output: Use wl_resource_get accessors for weston_output resources

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/screenshooter.c b/src/screenshooter.c
index 3a4f6c2..1fdfc9a 100644
--- a/src/screenshooter.c
+++ b/src/screenshooter.c
@@ -175,7 +175,8 @@
 		    struct wl_resource *output_resource,
 		    struct wl_resource *buffer_resource)
 {
-	struct weston_output *output = output_resource->data;
+	struct weston_output *output =
+		wl_resource_get_user_data(output_resource);
 	struct screenshooter_frame_listener *l;
 	struct wl_buffer *buffer = buffer_resource->data;