New base classes of TrackPanel just for the bitmap backing and overlay system...

... to make it reusable by other classes.

Also, don't create a second DC when repainting TrackPanel.

src/widgets/Overlay.h

@@ -0,0 +1,40 @@
+//
+//  Overlay.h
+//  Audacity
+//
+//  Created by Paul Licameli on 5/7/16.
+//
+//
+
+#ifndef __AUDACITY_OVERLAY__
+#define __AUDACITY_OVERLAY__
+
+#include <utility>
+
+class OverlayPanel;
+class wxDC;
+class wxRect;
+class wxSize;
+
+class Overlay
+{
+public:
+   virtual ~Overlay() = 0;
+
+   // nonvirtual wrapper
+   std::pair<wxRect, bool> GetRectangle(wxSize size);
+
+   // size passes the dimensions of the backing dc
+   // First member of pair is the rectangle that would be erased
+   // Second member of pair indicates whether the overlay is out of date
+   virtual std::pair<wxRect, bool> DoGetRectangle(wxSize size) = 0;
+
+   // Default implementation blits from backing store over GetRectangle().first
+   virtual void Erase(wxDC &dc, wxDC &src);
+
+   // Draw; dc.GetSize() tells you the total dimensions, and the panel is supplied
+   // as context
+   virtual void Draw(OverlayPanel &panel, wxDC &dc) = 0;
+};
+
+#endif