title: I have an SCNScene instance. How do I extract each SCNGeometry instance?
body: <p>In a <code>Metal</code> app written in <code>Swift</code> I want to extract the <code>SCNGeometry</code> instances from an <code>SCNScene</code> instance to go on and build <code>MDLMesh</code>'s. I see no API call that pulls out the geometry (similarly for cameras, and lights). How do I do this?</p>