renderer: ensure offscreen surface is created in main thread

Fixes render crash on some platforms.

app/task/render/render.h

@@ -24,6 +24,7 @@
 #include <QtConcurrent/QtConcurrent>
 
 #include "node/output/viewer/viewer.h"
+#include "render/backend/opengl/openglbackend.h"
 #include "task/task.h"
 
 OLIVE_NAMESPACE_ENTER
@@ -71,6 +72,8 @@ private:
 
   rational anchor_point_;
 
+  OpenGLBackend backend_;
+
 };
 
 OLIVE_NAMESPACE_EXIT