GLTFUnarchiver + GLTFSceneSource: add embedExternalImages option

[kohlmannj]

During development on an internal tool, a colleague and I needed an option to suppress GLTFSceneKit’s default behavior of embedding external image data into a SceneKit file.

This PR achieves that by adding a embedExternalImages option to GTFSceneSource constructor, which is true by default, thereby matching GLTFSceneKit’s current behavior.

When the setting is false, a GLTFUnarchiver class instance will return a URL to an image, as extracted from a .gltf file, rather than Image data.