[screenlocker] Render greeter backgrounds as black

The background of the whole screen locker architecture is black. During
starting the lockscreen there might be a frame with just the background.
To prevent flickering let's better use black instead of the default
white.

ksmserver/screenlocker/greeter/greeterapp.cpp

@@ -167,6 +167,7 @@ void UnlockApp::desktopResized()
         // create the view
         auto *view = new KQuickAddons::QuickViewSharedEngine();
         connect(view, &KQuickAddons::QuickViewSharedEngine::statusChanged, this, &UnlockApp::viewStatusChanged);
+        view->setColor(Qt::black);
 
         // first create KDeclarative, to be sure that it created a KIO Network Factory
         KDeclarative::KDeclarative declarative;