track: when length changes, force the new area to invalidate

The track limits cache invalidation signals to the length of the track. On some
platforms, if a block length was changed to the point that it changed the
track length, the track length would change AFTER the cache invalidation and
therefore never end up getting rendered.

This commit ensures that when a track's length changes, the new section is
invalidated regardless of ordering.

app/node/output/track/track.cpp

@@ -450,8 +450,13 @@ void TrackOutput::UpdateInOutFrom(int index)
 
   // Update track length
   if (new_track_length != track_length_) {
+    rational old_track_length = track_length_;
+
     track_length_ = new_track_length;
     emit TrackLengthChanged();
+
+    InvalidateCache(qMin(old_track_length, new_track_length),
+                    qMax(old_track_length, new_track_length));
   }
 }