force subview refresh on macOS 10.14

even on dark mode not all NSViews have their own layer, therefore don’t take that as a condition, because a child of such a view still might be layer-backed and need an explicit redraw.
Avoid spurious redraws for not visible windows (especiall important for not-yet fully constructed views or views during destruction)

src/osx/window_osx.cpp

@@ -1919,6 +1919,8 @@ void wxWindowMac::MacUpdateClippedRects() const
 bool wxWindowMac::MacDoRedraw( long time )
 {
     bool handled = false ;
+    if ( !IsShownOnScreen() )
+        return handled;
 
     wxRegion formerUpdateRgn = m_updateRegion;
     wxRegion clientUpdateRgn = formerUpdateRgn;