Bug 492458

There seems to be a new HDR image format gaining some traction. I haven't got a good understanding of them yet, so I am just writing down some very rough notes:

Gain Map spec draft by Adobe: https://helpx.adobe.com/nz/camera-raw/using/gain-map.html
This seems to be in the process of being standardized as ISO/CD 21496-1.

A website with some info on gain map: https://gregbenzphotography.com/hdr-photos/jpg-hdr-gain-maps-in-adobe-camera-raw/

## File format support:

Google's "Ultra HDR Image" format: https://developer.android.com/media/platform/hdr-image-format
This seems to be a format Google has developed to encode gain map data into JPEG in a backward-compatible way.
They also have a library for handling this: https://github.com/google/libultrahdr
(This seems to be slightly different from Adobe's JPEG gain map, but I haven't found the spec on that.)

Apple seems to use gain map in their HEIF photos (needs checking).

libavif and libjxl has experimental gain map reading and writing support API.

## Support in other software (as far as I know):

- Adobe Camera Raw seems to be able to output JPEG gain map images.
- Chrome is able to read and render such JPEG images (tested on Windows)
- Ultra HDR is supported on Android 14
- Google Pixel Camera can save Ultra HDR (tested on Pixel 6)

## Relevance to Krita:

- Krita may want to be able to import these files as HDR format.
- This format may be a good candidate for exporting HDR painting.