fixes jitters by using signals/slots to pass image buffers around

Updating values rapidly would cause strange jitters as a
byproduct of the viewer trying to update from the renderer while
it was still working. Rather than the viewer trying to access the
the renderer, we now send textures in the initial update signal
to keep everything synchronized.

app/render/backend/renderworker.h

@@ -26,7 +26,7 @@ public slots:
 
   void Render(NodeDependency path);
 
-  virtual void RenderAsSibling(NodeDependency dep) = 0;
+  virtual QVariant RenderAsSibling(NodeDependency dep) = 0;
 
 signals:
   void RequestSibling(NodeDependency path);