renderer: ensure offscreen surface is created in main thread

Fixes render crash on some platforms.

app/render/backend/opengl/openglbackend.cpp

@@ -25,10 +25,17 @@
 OLIVE_NAMESPACE_ENTER
 
 OpenGLBackend::OpenGLBackend(QObject* parent) :
-  RenderBackend(parent),
-  proxy_(nullptr)
+  RenderBackend(parent)
 {
+  proxy_ = new OpenGLProxy();
 
+  QThread* proxy_thread = new QThread();
+  proxy_thread->start(QThread::IdlePriority);
+  proxy_->moveToThread(proxy_thread);
+
+  if (!proxy_->Init()) {
+    ClearProxy();
+  }
 }
 
 OpenGLBackend::~OpenGLBackend()
@@ -40,19 +47,6 @@ OpenGLBackend::~OpenGLBackend()
 
 RenderWorker *OpenGLBackend::CreateNewWorker()
 {
-  if (!proxy_) {
-    proxy_ = new OpenGLProxy();
-
-    QThread* proxy_thread = new QThread();
-    proxy_thread->start(QThread::IdlePriority);
-    proxy_->moveToThread(proxy_thread);
-
-    if (!proxy_->Init()) {
-      ClearProxy();
-      return nullptr;
-    }
-  }
-
   return new OpenGLWorker(this, proxy_);
 }
 

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);