[>64k:layout:MultipleSubst] Implement format 2

Implements part of harfbuzz/boring-expansion-spec#32

Subset is NOT updated to lower format.

src/OT/Layout/GSUB/MultipleSubst.hh

@@ -14,6 +14,9 @@ struct MultipleSubst
   union {
   HBUINT16				format;         /* Format identifier */
   MultipleSubstFormat1_2<SmallTypes>	format1;
+#ifndef HB_NO_BORING_EXPANSION
+  MultipleSubstFormat1_2<MediumTypes>	format2;
+#endif
   } u;
 
   public:
@@ -25,10 +28,15 @@ struct MultipleSubst
     if (unlikely (!c->may_dispatch (this, &u.format))) return_trace (c->no_dispatch_return_value ());
     switch (u.format) {
     case 1: return_trace (c->dispatch (u.format1, std::forward<Ts> (ds)...));
+#ifndef HB_NO_BORING_EXPANSION
+    case 2: return_trace (c->dispatch (u.format2, std::forward<Ts> (ds)...));
+#endif
     default:return_trace (c->default_return_value ());
     }
   }
 
+  /* TODO This function is unused and not updated to 24bit GIDs. Should be done by using
+   * iterators. While at it perhaps using iterator of arrays of hb_codepoint_t instead. */
   bool serialize (hb_serialize_context_t *c,
                   hb_sorted_array_t<const HBGlyphID16> glyphs,
                   hb_array_t<const unsigned int> substitute_len_list,
@@ -43,6 +51,9 @@ struct MultipleSubst
     default:return_trace (false);
     }
   }
+
+  /* TODO subset() should choose format. */
+
 };