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/common/sizer.cpp

@@ -111,7 +111,7 @@ float wxSizerFlags::DoGetDefaultBorderInPx()
     if ( s_defaultBorderInPx.HasChanged(win) )
     {
         s_defaultBorderInPx.SetAtNewDPI(
-            (float)(5 * (win ? win->GetContentScaleFactor() : 1)));
+            (float)(5 * (win ? win->GetDPIScaleFactor() : 1.0)));
     }
     return s_defaultBorderInPx.Get();
 }