[subset/cff1] Speed up for retaingids

src/hb-subset-cff1.cc

@@ -487,10 +487,17 @@ struct cff_subset_plan {
 
     unsigned int glyph;
     unsigned num_glyphs = plan->num_output_glyphs ();
+    auto it = hb_iter (plan->new_to_old_gid_list);
+    if (it->first == 0) it++;
     for (glyph = 1; glyph < num_glyphs; glyph++)
     {
-      hb_codepoint_t  old_glyph;
-      if (!plan->old_gid_for_new_gid (glyph, &old_glyph))
+      hb_codepoint_t old_glyph;
+      if (glyph == it->first)
+      {
+	old_glyph = it->second;
+	it++;
+      }
+      else
       {
 	/* Retain the SID for the old missing glyph ID */
 	old_glyph = glyph;