[gpos] Break cycle-of-2 when cursive attaching

Fixes https://github.com/harfbuzz/harfbuzz/issues/2469

src/hb-ot-layout-gpos-table.hh

@@ -1692,6 +1692,12 @@ struct CursivePosFormat1
     else
       pos[child].x_offset = x_offset;
 
+    /* If parent was attached to child, break them free.
+     * https://github.com/harfbuzz/harfbuzz/issues/2469
+     */
+    if (unlikely (pos[parent].attach_chain() = -pos[child].attach_chain()))
+      pos[parent].attach_chain() = 0;
+
     buffer->idx++;
     return_trace (true);
   }