Bug 2022 Mixer Board does not automatically update

OnTrackSetChanged sets mUpToDate to false, which assumes that
MixerBoard::OnPaint(evt) will be called, but on Linux, the event
appears to be eaten by MixerTrackCluster::OnPaint and TrackClusters
were not updated.

src/MixerBoard.cpp

@@ -1372,6 +1372,7 @@ void MixerBoard::OnTrackSetChanged(wxEvent &evt)
 {
    evt.Skip();
    mUpToDate = false;
+   UpdateTrackClusters();
    Refresh();
 }