fix transparency problems on macOS

under mojave any backgroundColor destroys transparency

src/osx/cocoa/nonownedwnd.mm

@@ -914,6 +914,11 @@ bool wxNonOwnedWindowCocoaImpl::SetTransparent(wxByte alpha)
 
 bool wxNonOwnedWindowCocoaImpl::SetBackgroundColour(const wxColour& col )
 {
+    // only set the native background color if the toplevel window's
+    // background is not supposed to be transparent, otherwise the
+    // transparency is lost
+    if( GetWXPeer()->GetBackgroundStyle() != wxBG_STYLE_TRANSPARENT &&
+       !(GetWXPeer()->GetWindowStyle() & wxFRAME_SHAPED) )
         [m_macWindow setBackgroundColor:col.OSXGetNSColor()];
     return true;
 }

src/osx/nonownedwnd_osx.cpp

@@ -152,7 +152,6 @@ bool wxNonOwnedWindow::Create(wxWindow *parent,
     wxWindowCreateEvent event(this);
     HandleWindowEvent(event);
 
-    if ( GetBackgroundStyle() != wxBG_STYLE_TRANSPARENT )
     SetBackgroundColour(wxSystemSettings::GetColour( wxSYS_COLOUR_APPWORKSPACE ));
 
     if ( parent )