[subset] add a new closure call to hb-ot-layout that can compute the closure over multiple lookups.

src/hb-ot-shape.cc

diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index c20b110..36e0bf9 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -957,15 +957,7 @@
 
   hb_set_t *lookups = hb_set_create ();
   hb_ot_shape_plan_collect_lookups (shape_plan, HB_OT_TAG_GSUB, lookups);
-
-  /* And find transitive closure. */
-  hb_set_t *copy = hb_set_create ();
-  do {
-    copy->set (glyphs);
-    for (hb_codepoint_t lookup_index = HB_SET_VALUE_INVALID; hb_set_next (lookups, &lookup_index);)
-      hb_ot_layout_lookup_substitute_closure (font->face, lookup_index, glyphs);
-  } while (!copy->is_equal (glyphs));
-  hb_set_destroy (copy);
+  hb_ot_layout_lookups_substitute_closure (font->face, lookups, glyphs);
 
   hb_set_destroy (lookups);