Use GetContentScaleFactor() to obtain the scale of the back buffer

GetContentScaleFactor() is implemented on all platforms and therefore
can be used to obtain the proper scale not only under wxOSX.

src/propgrid/propgrid.cpp

@@ -4650,12 +4650,7 @@ void wxPropertyGrid::OnResize( wxSizeEvent& event )
 
     if ( !HasExtraStyle(wxPG_EX_NATIVE_DOUBLE_BUFFERING) )
     {
-        // Scaled bitmaps only work on Mac currently
-#ifdef __WXOSX_COCOA__
         double scaleFactor = GetContentScaleFactor();
-#else
-        double scaleFactor = 1.0;
-#endif
         int dblh = (m_lineHeight*2);
         if ( !m_doubleBuffer )
         {