Support saving pixel formats other than BGRA32 (PDN 6.0+)

As per the `// TODO` I added in `JpegXLSave` at https://github.com/0xC0000054/pdn-jpegxl/pull/11/changes#diff-723734236d7e5fa9d777fea698b6da1d36b3ffb9ba8427820c0f3ae401161e34R38

This won't be necessary until PDN 6.0, which is when I'm planning to add native support for some more pixel formats, such as RGBA64.
