[aat] Set debug lookup_index
diff --git a/src/hb-aat-layout-common-private.hh b/src/hb-aat-layout-common-private.hh
index a421a4c..e0f59a4 100644
--- a/src/hb-aat-layout-common-private.hh
+++ b/src/hb-aat-layout-common-private.hh
@@ -713,6 +713,8 @@
     sanitizer.start_processing ();
   }
 
+  inline void set_lookup_index (unsigned int i) { lookup_index = i; }
+
   inline ~hb_aat_apply_context_t (void)
   {
     sanitizer.end_processing ();