[subset/glyf] Add a const

src/OT/glyf/SubsetGlyph.hh

@@ -22,7 +22,7 @@ struct SubsetGlyph
 
   bool serialize (hb_serialize_context_t *c,
 		  bool use_short_loca,
-		  const hb_subset_plan_t *plan)
+		  const hb_subset_plan_t *plan) const
   {
     TRACE_SERIALIZE (this);
 

src/OT/glyf/glyf.hh

@@ -129,7 +129,7 @@ struct glyf
         padded_offsets[i] = glyphs[i].length ();
     }
 
-    bool result = glyf_prime->serialize (c->serializer, glyphs.writer (), use_short_loca, c->plan);
+    bool result = glyf_prime->serialize (c->serializer, glyphs.iter (), use_short_loca, c->plan);
     if (c->plan->normalized_coords && !c->plan->pinned_at_default)
       _free_compiled_subset_glyphs (glyphs);