[repacker] always duplicate classDef2 when splitting a PairPos2.

Splits are done in a way that it shouldn't be possible to share the classDef2 between split PairPos2's so pre-emptively duplicate it.

src/graph/pairpos-graph.hh

@@ -381,6 +381,7 @@ struct PairPosFormat2 : public OT::Layout::GPOS_impl::PairPosFormat2_4<SmallType
     class_def_link->objidx = class_def_2_id;
     class_def_link->position = 10;
     split_context.c.graph.vertices_[class_def_2_id].parents.push (pair_pos_prime_id);
+    split_context.c.graph.duplicate (pair_pos_prime_id, class_def_2_id);
 
     return pair_pos_prime_id;
   }

src/test-repacker.cc

@@ -1211,7 +1211,7 @@ populate_serializer_with_large_pair_pos_2 (hb_serialize_context_t* c,
                                 5 + num_class_1 - 1,
                                 c);
 
-    pair_pos_2[i] = add_pair_pos_2 (i * num_class_1,
+    pair_pos_2[i] = add_pair_pos_2 (1 + i * num_class_1,
                                     coverage[i],
                                     class_def_1[i], num_class_1,
                                     class_def_2[i], num_class_2,