将SCNView渲染到屏幕外缓冲区以生成图像

I have a SCNView, it renders my scene. However, I want to get a couple of screenshots at different time points when I save. My best guess at this is to create a SCNRenderer and render the scene specifying different times. I have tried, but my image just comes back as blank, here's my code, any ideas?:I have a SCNView, it renders my scene. However,