OSX 1/2 refresh frame-limiting hack

src/visual/TuningCanvas.cpp

@@ -36,6 +36,9 @@ TuningCanvas::~TuningCanvas() {
 
 void TuningCanvas::OnPaint(wxPaintEvent& WXUNUSED(event)) {
     wxPaintDC dc(this);
+#ifdef __APPLE__    // force half-rate?
+    glFinish();
+#endif
     const wxSize ClientSize = GetClientSize();
 
     glContext->SetCurrent(*this);