Camera Y axis inversion inelegant in "Nested Scenes" project

[halmueller]

SceneKit and SpriteKit use different Y axis directions. That means a camera Y axis inversion is needed to get the SceneKit scene to render right side up in the SpriteKit 3D Node.

The spinningShipScene() function has a flip of the camera to make this work, but having this change hardwired in the scene is ugly.