disambiguated whether NodeParam::parent() would return a Node* or a QObject*

Old code assumes that a NodeParam's parent will always be a Node. The function
has been separated off and tweaked in the event that this is not the case.

app/render/backend/renderworker.cpp

@@ -171,7 +171,7 @@ QList<NodeInput*> RenderWorker::ProcessNodeInputsForTime(Node *n, const TimeRang
 QVariant RenderWorker::ProcessNodeNormally(const NodeDependency& dep)
 {
   NodeOutput* output = dep.node();
-  Node* node = dep.node()->parent();
+  Node* node = dep.node()->parentNode();
 
   //qDebug() << "Processing" << node->id();