[API] Make set_user_data() functions take a replace parameter
We need this to set data on objects safely without worrying that some
other thread unsets it by setting it at the same time.
diff --git a/src/hb-buffer.h b/src/hb-buffer.h
index a43a8d1..a5efce3 100644
--- a/src/hb-buffer.h
+++ b/src/hb-buffer.h
@@ -75,7 +75,8 @@
hb_buffer_set_user_data (hb_buffer_t *buffer,
hb_user_data_key_t *key,
void * data,
- hb_destroy_func_t destroy);
+ hb_destroy_func_t destroy,
+ hb_bool_t replace);
void *
hb_buffer_get_user_data (hb_buffer_t *buffer,
