region: add dimension and value_type constants

for interoperability with vector and point classes

region.hpp

@@ -34,6 +34,9 @@ namespace util {
         typedef T position_type;
         typedef T size_type;
 
+        static constexpr size_t dimension = 2u;
+        typedef T value_type;
+
         position_type x, y;
         size_type w, h;