How can I convert a uiimage or cgimage to a cvpixelbuffer?

Question

I know how to do it the other way around, but cannot figure out how to get a core video pixelbuffer from an image.

Bonus points for a code sample showing the use of pixelBufferPools with an AVAssetWriterInputPixelBufferAdaptor. I have not found any samples or any real use case in the docs.