Add wxWindow::GetDPIScaleFactor()

This function replaces some uses of GetContentScaleFactor(), where a
factor greater than 1 must be used even under the platforms not doing
any logical/physical pixel mapping, such as MSW.

For now GetContentScaleFactor() is still unchanged, but it will return 1
for such platforms in the future and adding GetDPIScaleFactor() allows
to avoid changing the behaviour of the code which relied on its current
behaviour.

src/propgrid/propgrid.cpp

@@ -4610,7 +4610,7 @@ void wxPropertyGrid::OnResize( wxSizeEvent& event )
 
     if ( !HasExtraStyle(wxPG_EX_NATIVE_DOUBLE_BUFFERING) )
     {
-        double scaleFactor = GetContentScaleFactor();
+        double scaleFactor = GetDPIScaleFactor();
         int dblh = (m_lineHeight*2);
         if ( !m_doubleBuffer )
         {