add WFObj::VertexRef::operator<()

WFObj.cc

@@ -326,3 +326,12 @@ bool WFObj::loadfile(const char *path, Buffer *buff)
 {
     /* TODO */
 }
+
+bool WFObj::VertexRef::operator<(const VertexRef & other)
+{
+    if (vertex != other.vertex)
+        return vertex < other.vertex;
+    if (texture != other.texture)
+        return texture < other.texture;
+    return normal < other.normal;
+}

WFObj.h

@@ -51,6 +51,7 @@ protected:
             int vertex;
             int texture;
             int normal;
+            bool operator<(const VertexRef & other);
     };
 
     class Face