JXL improvements

Highlights of the patch:
- Supersede MR !249
- 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).

src/imageformats/jxl_p.h

@@ -51,6 +51,7 @@ private:
     bool countALLFrames();
     bool decode_one_frame();
     bool rewind();
+    bool decodeBoxes();
 
     enum ParseJpegXLState {
         ParseJpegXLError = -1,
@@ -77,6 +78,7 @@ private:
 
     QImage m_current_image;
     QColorSpace m_colorspace;
+    QByteArray m_xmp;
 
     QImage::Format m_input_image_format;
     QImage::Format m_target_image_format;