[repacker] use possibly updated root idx after isolate_subgraph.

isolate_subgraph can change the root indices in some cases. So operations after the isolation need to use the roots from the output of isolate_subgraph.
diff --git a/src/hb-repacker.hh b/src/hb-repacker.hh
index 14e1056..288bae5 100644
--- a/src/hb-repacker.hh
+++ b/src/hb-repacker.hh
@@ -1048,7 +1048,8 @@
     hb_set_t roots;
     roots.add (root);
     sorted_graph.isolate_subgraph (roots);
-    sorted_graph.move_to_new_space (root);
+    for (unsigned new_root : roots)
+      sorted_graph.move_to_new_space (new_root);
     return true;
   return false;