Added per-pixel alpha blending capability to wxOverlay under wxMSW

By default, wxMSW overlay uses a constant alpha for the window's opacity which
can be changed by the added wxOverlay::SetOpacity() function. For some applications,
that's all they need. But for others, per-pixel alpha blending is a requirement.
This capability can be enabled by calling SetOpacity(-1) before initializing the
overlay. But the drawing must be done via a graphics context because the standard DC
under MSW does not support alpha drawing.

Also request that the overlay window be positioned below any floating windows
on the target (if any).

include/wx/overlay.h

@@ -34,6 +34,8 @@ public:
 
     bool IsNative() const;
 
+    void SetOpacity(int alpha);
+
 private:
     friend class WXDLLIMPEXP_FWD_CORE wxDCOverlay;
 

include/wx/private/overlay.h

@@ -38,6 +38,7 @@ public:
     virtual void EndDrawing(wxDC* dc) = 0;
     virtual void Clear(wxDC* dc) = 0;
     virtual void Reset() = 0;
+    virtual void SetOpacity(int WXUNUSED(alpha)) { }
 };
 
 #endif // _WX_PRIVATE_OVERLAY_H_