[layout] Add large_int to Types

src/OT/Layout/Common/CoverageFormat2.hh

@@ -67,10 +67,10 @@ struct CoverageFormat2_4
 
   unsigned get_population () const
   {
-    unsigned ret = 0;
+    typename Types::large_int ret = 0;
     for (const auto &r : rangeRecord)
-      ret += r.get_population (); // TODO Overflow
-    return ret;
+      ret += r.get_population ();
+    return ret > UINT_MAX ? UINT_MAX : (unsigned) ret;
   }
 
   template <typename Iterator,

src/OT/Layout/types.hh

@@ -34,6 +34,7 @@ namespace Layout {
 
 struct SmallTypes {
   static constexpr unsigned size = 2;
+  using large_int = uint32_t;
   using HBUINT = HBUINT16;
   using HBGlyphID = HBGlyphID16;
   using Offset = Offset16;
@@ -47,6 +48,7 @@ struct SmallTypes {
 
 struct MediumTypes {
   static constexpr unsigned size = 3;
+  using large_int = uint64_t;
   using HBUINT = HBUINT24;
   using HBGlyphID = HBGlyphID24;
   using Offset = Offset24;