A simple JPEG XL support implementation for doublecmd.
Based on src/platform/unix/udcreadwebp.pas by Alexander Koblov

Provides thumbnails for JXL pictures.
Allows for viewing JXL pictures in the internal viewer.

Works fine with libjxl 0.8.2 and doublecmd 1.0.11

Currently:
- For the animated pictures, processes only the first frame;
- When reading a picture, attempts to convert it to rgba8bpc;
- For libjxl loading, only the Linux ".so" library file extensions are used;
- When InternalCheck() is called,  always returns true.

To compile doublecmd with this:
- Copy udcreadjxl.pas to src/platform/unix/
- Add uDCReadJXL to the list within src/platform/uosforms.pas next to where uDCReadWebP is

If you want to compile doublecmd for Windows, you can try the following:
1. Change the library extensions within udcreadjxl.pas to ".dll";
2. Copy udcreadjxl.pas to src/platform/win/
3. Add uDCReadJXL to the list within src/platform/uosforms.pas under {$IF DEFINED(MSWINDOWS)}