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.

[kohlmannj]

I’ve self-reviewed with a few comments here. Please let me know if you have any additional feedback.

[kohlmannj]

Please note that I’m new to Swift, and my most recent experience with typed languages is TypeScript. Unfortunately I couldn’t declare this as an array of Image | URL types. If Any is unacceptable, I would consider this instead:

Suggested change

	private var images: [Any?] = []
	private var images: [Image?] = []
	private var imageUrls: [URL?] = []

[kohlmannj]

Similar to the above comment regarding Any, I am willing to revise this part of the code to avoid Any as the return type. I would consider factoring out the shared logic here in loadImage() and then create a second private func loadImageUrl(index: Int) throws -> URL method.

[kohlmannj]

This is the most awkward part of this revision. As indicated in the comments, I’m essentially using loadImageFile() as a test to see if the file exists on-disk. There are a few ways to improve this:

• Add a helper function to simply check if the image URL exists on-disk when self.embedExternalImages is false
• Call loadImageFile() only when self.embedExternalImages is true

[kohlmannj]

Same comment regarding Any usage as the above comments (first comment, second comment).