Correct template parameters to std::iterator base of TrackIter...

... so that std::reverse_iterator<TrackIter<T>> works right, not making a
subtle error of return of reference to a temporary in its operator * with some
compilers.

src/Track.h

@@ -818,7 +818,14 @@ template <
    typename TrackType // Track or a subclass, maybe const-qualified
 > class TrackIter
    : public std::iterator<
-      std::bidirectional_iterator_tag, TrackType *const
+      std::bidirectional_iterator_tag,
+      TrackType *const,
+      ptrdiff_t,
+      // pointer is void to disable operator -> in the reverse_iterator...
+      void,
+      // ... because what operator * returns is really a value type,
+      // so you can't take its address
+      TrackType *const
    >
 {
 public: