Use wl_resource_get_user_data for weston_surface resources

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/src/input.c b/src/input.c
index 5463d78..9b6d475 100644
--- a/src/input.c
+++ b/src/input.c
@@ -1108,7 +1108,7 @@
 	struct weston_surface *surface = NULL;
 
 	if (surface_resource)
-		surface = surface_resource->data;
+		surface = wl_resource_get_user_data(surface_resource);
 
 	if (pointer->focus == NULL)
 		return;