NOTE: Should we expose Engine::getEntityManager() to help with issues
like this? e.g. it would allow gltfio to extract it from the engine
rather than consuming an additional argument.
This fixes an entity id collision that I saw when building gltfio into
its own DSO (e.g. on Android). Since it is a separate executable it has
its own EntityManager singleton. This id collision was causing unwanted
entities to be added to the scene.
a7ab0d98fa