add share option to pop_pack() (#2176)

as a solution for #2164

src/hb-serialize.hh

@@ -195,7 +195,12 @@ struct hb_serialize_context_t
     obj->fini ();
     object_pool.free (obj);
   }
-  objidx_t pop_pack ()
+
+  /* Set share to false when an object is unlikely sharable with others
+   * so not worth an attempt, or a contiguous table is serialized as
+   * multiple consecutive objects in the reverse order so can't be shared.
+   */
+  objidx_t pop_pack (bool share=true)
   {
     object_t *obj = current;
     if (unlikely (!obj)) return 0;
@@ -211,12 +216,16 @@ struct hb_serialize_context_t
       return 0;
     }
 
-    objidx_t objidx = packed_map.get (obj);
+    objidx_t objidx;
+    if (share)
+    {
+      objidx = packed_map.get (obj);
       if (objidx)
       {
 	obj->fini ();
 	return objidx;
       }
+    }
 
     tail -= len;
     memmove (tail, obj->head, len);
@@ -231,7 +240,7 @@ struct hb_serialize_context_t
 
     objidx = packed.length - 1;
 
-    packed_map.set (obj, objidx);
+    if (share) packed_map.set (obj, objidx);
 
     return objidx;
   }