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

Implements rest of https://github.com/be-fonts/boring-expansion-spec/issues/32

Subset does NOT lower format.

src/OT/Layout/GSUB/AlternateSubst.hh

@@ -14,6 +14,9 @@ struct AlternateSubst
   union {
   HBUINT16				format;         /* Format identifier */
   AlternateSubstFormat1_2<SmallTypes>	format1;
+#ifndef HB_NO_BORING_EXPANSION
+  AlternateSubstFormat1_2<MediumTypes>	format2;
+#endif
   } u;
   public:
 
@@ -24,10 +27,15 @@ struct AlternateSubst
     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> alternate_len_list,
@@ -42,6 +50,9 @@ struct AlternateSubst
     default:return_trace (false);
     }
   }
+
+  /* TODO subset() should choose format. */
+
 };
 
 }