Improve iOS image setting as images can only be loaded locally

[will3942]

iOS does not support loading an image from HTTP, this ensures the file both exists (to avoid a panic) and is a file:// URL

[Sinono3]

Hm. Because we are not 100% sure what behavior the OS implements, I'm not comfortable with adding this std::fs check as it is currently. For example, it was recently found out that Base64-encoded images work on MacOS. This PR removes that functionality by assuming the URL must start with file://. May I get write access to the branch? I'd like to add a simple if around what you implemented, to check if the URL starts with file:// or not before making the std::fs call. I think this is the best way to reconcile safety and functionality.

[will3942]

@Sinono3 The reason why I enforced the check is that otherwise a panic occurs on iOS if you pass in a HTTP URL which is not a great experience. If we can load via Base64-encoding then we should definitely allow that path, perhaps an additional check to ensure HTTP URLs cannot be passed could be beneficial here too?

[Sinono3]

Is it possible for you to check if base64-encoded images work in iOS? If so, then we can do what you suggested: on iOS, check whether the provided URL is HTTP, and if so, return a null image. (We could print a warning here as well.)

This issues convinces me that the entire approach to platform differences are handled in souvlaki should be changed. The current approach is prone to mistakes.

[Sinono3]

@will3942 Sorry, I missed the fact that the actual iOS implementation does use base64, my bad. To make myself clear, I meant base64-encoded data URL images, not base64-encoded images, which the implementation already uses.

I'm currently rewriting a lot of the macOS implementation, and I have a few questions for you regarding the iOS functions:

1. In mp_artwork, why use MPMediaItemArtwork.init(image: UIImage)? It's deprecated. The original MPMediaItemArtwork.init(boundsSize: CGSize, requestHandler: (CGSize) -> UIImage) should (theoretically) work fine on iOS. Has this been different in your experience?
2. In load_image_from_url, why not directly use a pointer to load the information instead of using base64? Maybe doing it that way may reduce copies and memory usage. I know that that the base64 method is safer though. I could try to implement the pointer method but sadly I have no way to verify it's functioning. (I have no iOS device.) What about UIImage.init(contentsOfFile:)
   , isn't this simpler?