renderer: ensure offscreen surface is created in main thread

Fixes render crash on some platforms.

app/render/backend/renderbackend.h

@@ -41,6 +41,8 @@ public:
 
   virtual ~RenderBackend() override;
 
+  void Close();
+
   void SetViewerNode(ViewerOutput* viewer_node);
 
   void SetUpdateWithGraph(bool e);
@@ -76,8 +78,6 @@ public slots:
 protected:
   virtual RenderWorker* CreateNewWorker() = 0;
 
-  void Close();
-
 private:
   void CopyNodeInputValue(NodeInput* input);
   Node *CopyNodeConnections(Node *src_node);