JXL improvements

Highlights of the patch:

• Supersede MR !249 (closed)
• Added FP16 and FP32 images support thus preserving HDR values (read / write, required libjxl 0.9+).
• Added Gray8 and Gray16 support (read / write).
• Indexed images are saved as Gray8 when palette is gray scale.
• Binary images are saved as Gray8 (does JXL natively support binary images?).
• Simplified writing process by partially removing the use of additional buffers.
• Added XMP metadata support by decoding/encoding Boxes.
• Changed maximum image size in pixels in accordance with JXL feature level 5 (still limited to 256 megapixels).

Compatibility:

• Older versions of this plugin load FP images correctly as UINT16 (obviously losing HDR info).
• HDR images saved with this patch are also loaded correctly by Gimp and Photoshop.
• Grayscale images saved with this patch are also loaded correctly by Gimp and Photoshop.

Compilation modifiers for cmake file:

• JXL_HDR_PRESERVATION_DISABLED: disable the FP support (behaves like previous versions).
• JXL_DECODE_BOXES_DISABLED: disable metadata reading (behaves like previous versions).