Gain maps in JPEG XL files

[jonsneyers]

Currently we don't define a way to embed gain maps in JPEG XL files. There is currently an ongoing standardization effort by ISO/TC 42 (ISO 21496-1, https://www.iso.org/standard/86775.html?browse=tc) that could also be relevant for JPEG XL.

JPEG XL supports HDR natively, so there is no real need to use gain maps to represent HDR image data (unlike formats that are limited to 8-bit precision). But inverse gain maps (i.e. describing a custom tone mapping) could still be useful.

This is also somewhat related to #3504, in case the JPEG XL file represents a recompressed JPEG with a gain map.

This thread can be used to discuss what would be the best way to represent gain maps in JPEG XL files, what changes/additions to ISO/IEC 18181-2 (JPEG XL File Format) would be desired, and what changes/additions to libjxl would be desired.