Discard all deleted before constructing the Scene

The Scene connects to the deletedRemoved signal and tries to remove the
Deleted from it's list of Windows, which will fail because a SceneWindow
doesn't get created for an existing Deleted in the Scene setup.

It doesn't make sense to create a SceneWindow for a Deleted as there is
no pixmap around anyway. Thus the best approach is to ensure that there
are no Deleted by just discarding all prior to creating the Scene.

BUG: 333275
REVIEW: 117557

composite.cpp

@@ -175,6 +175,12 @@ void Compositor::slotCompositingOptionsInitialized()
         cm_selection->claim(true);   // force claiming
         cm_selection->owning = true;
     }
+
+    // There might still be a deleted around, needs to be cleared before creating the scene (BUG 333275)
+    while (!Workspace::self()->deletedList().isEmpty()) {
+        Workspace::self()->deletedList().first()->discard();
+    }
+
     switch(options->compositingMode()) {
     case OpenGLCompositing: {
         qDebug() << "Initializing OpenGL compositing";