This contains two updates that should speed up the loading of JXL images.
- Use an image-out callback with libjxl to convert pieces of pixel data. This is called by decoder threads and should be faster than doing the whole image in one thread at the end.
requested_sizeis defined and small enough (e.g for a thumbnail), try to load just the 1/8th DC image. This only works if the JXL file is encoded progressively. Sadly this isn't the default because it usually increases the file size slightly.