Implement GetDPIScaleFactor() more directly for wxMSW too

Use the existing GetDPI() implementation instead of forwarding to
wxDisplay.

include/wx/msw/window.h

@@ -100,6 +100,8 @@ public:
     virtual bool Reparent(wxWindowBase *newParent) wxOVERRIDE;
 
     virtual wxSize GetDPI() const wxOVERRIDE;
+    virtual double GetDPIScaleFactor() const wxOVERRIDE;
+
 
     virtual void WarpPointer(int x, int y) wxOVERRIDE;
     virtual bool EnableTouchEvents(int eventsMask) wxOVERRIDE;

src/msw/window.cpp

@@ -105,6 +105,7 @@
 #include "wx/notebook.h"
 #include "wx/listctrl.h"
 #include "wx/dynlib.h"
+#include "wx/display.h"
 
 #include <string.h>
 
@@ -4858,6 +4859,11 @@ wxSize wxWindowMSW::GetDPI() const
     return dpi;
 }
 
+double wxWindowMSW::GetDPIScaleFactor() const
+{
+    return GetDPI().y / (double)wxDisplay::GetStdPPIValue();
+}
+
 void wxWindowMSW::MSWUpdateFontOnDPIChange(const wxSize& newDPI)
 {
     if ( m_font.IsOk() )