Merge branch 'display-ppi-from-scaling-factor'

Determine display DPI from scaling factor instead of trying to compute
it from the physical display dimensions.

See https://github.com/wxWidgets/wxWidgets/pull/2015

Closes #18855.

include/wx/osx/window.h

@@ -121,6 +121,7 @@ public:
 #endif // wxUSE_HOTKEY
 
     virtual wxSize GetDPI() const wxOVERRIDE;
+    virtual double GetDPIScaleFactor() const wxOVERRIDE;
 
 #if wxUSE_DRAG_AND_DROP
     virtual void SetDropTarget( wxDropTarget *dropTarget ) wxOVERRIDE;