change VertexRef elements to unsigned values to fix compile warnings

WFObj.h

@@ -92,9 +92,9 @@ protected:
     {
         public:
             VertexRef() : vertex(0), texture(0), normal(0) {}
-            int vertex;
-            int texture;
-            int normal;
+            size_t vertex;
+            size_t texture;
+            size_t normal;
             bool operator<(const VertexRef & other) const;
     };