use common contentScaleFactor also for DPI on macOS

and give a reasonable default for missing tlw

src/osx/cocoa/window.mm

@@ -2648,7 +2648,10 @@ void wxWidgetCocoaImpl::SetVisibility( bool visible )
 double wxWidgetCocoaImpl::GetContentScaleFactor() const
 {
     NSWindow* tlw = [m_osxView window];
-    return [tlw backingScaleFactor];
+    if ( tlw )
+        return [tlw backingScaleFactor];
+    else
+        return wxOSXGetMainScreenContentScaleFactor();
 }
 
 // ----------------------------------------------------------------------------

src/osx/window_osx.cpp

@@ -2636,13 +2636,7 @@ wxSize wxWindowMac::GetDPI() const
 // on mac ContentScale and DPIScale are identical
 double wxWindowMac::GetDPIScaleFactor() const
 {
-    double scaleFactor;
-    if ( wxNonOwnedWindow* tlw = MacGetTopLevelWindow() )
-        scaleFactor = tlw->GetContentScaleFactor();
-    else
-        scaleFactor = wxOSXGetMainScreenContentScaleFactor();
-
-    return scaleFactor;
+    return GetContentScaleFactor();
 }