Define TrackPanelOverlay, use it to reimplement play indicator and cursor...

... but scrub speed display is disabled

src/TrackPanelOverlay.cpp

@@ -0,0 +1,40 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+TrackPanelOverlay.cpp
+
+Paul Licameli split from TrackPanel.cpp
+
+**********************************************************************/
+
+#include "TrackPanelOverlay.h"
+
+#include <wx/dc.h>
+
+TrackPanelOverlay::~TrackPanelOverlay()
+{
+}
+
+std::pair<wxRect, bool> TrackPanelOverlay::GetRectangle(wxSize size)
+{
+   auto result = DoGetRectangle(size);
+#ifdef __WXMAC__
+   // On OSX, if a HiDPI resolution is being used, a vertical line will actually take up
+   // more than 1 pixel (even though it is drawn as 1), so we restore the surrounding
+   // pixels as well.  (This is because the wxClientDC doesn't know about the scaling.
+   result.first.Inflate(1, 0);
+#endif
+   return result;
+}
+
+void TrackPanelOverlay::Erase(wxDC &dc, wxDC &src)
+{
+   wxRect rect(dc.GetSize());
+   rect.Intersect(src.GetSize());
+   auto smallRect(GetRectangle(src.GetSize()).first);
+   rect.Intersect(smallRect);
+   if (!rect.IsEmpty())
+      dc.Blit(rect.x, rect.y, rect.width, rect.height,
+              &src, rect.x, rect.y);
+}