zstd media files

[zmm147]

Hi, I wanted to thank you for open-sourcing this project. I've been using it in my own project, and it's been really helpful. However, I came across an issue with the latest Anki .anki21b apkg file. The media files inside are compressed using zstd, and simply renaming the file doesn't fix the problem. I've managed to resolve it myself, but I thought I should report this so that others might be aware of it. Once again, thank you for making this project open-source.

[clejacquet]

Hi, thank you very much for letting me know, it's really appreciated.

I only tested on one or two .mp3 files when adding support for .anki21b, and surprisingly my audio reader app was able to read the compressed files, so I did not notice. After checking with other .jpg and .mp3 media files I ran into the same issue.

I made a quick fix in #5 and will test more later before merging, but it seems to work on my end. So far I haven't given media files a user-friendly API so I tried to improve it a little bit. Now, instead of providing the filename mapping, media files are directly renamed (and decompressed if the apkg file format is .anki21b). The root directory containing all renamed (and decompressed) files is provided by the field mediaRoot:

ApkgIOSwift/Sources/ApkgIO/ApkgIO.swift

Line 357 in 3c40c00

public let mediaRoot: URL

I also added a parameter to the parse function to enable or disable filename resolution (enabled by default). If enabled, the mediaMapping field is pretty much useless, but I don't want to break compatibility if some people already have some working code that uses it.