Request: support for JXL gain maps

Support for the JPEG XL (**JXL**) file format has returned to Chrome Canary (v146, under a developer flag: **chrome://flags/#enable-jxl-image-format**). This is very exciting, as JXL offers support for progressive downloading (which should offer a much better experience under slow internet), no further loss when transcoding from existing JPG images (which should be ideal for re-compressing today's JPG gain maps), reportedly faster encode speeds, and in some cases smaller images than AVIF.

This request is more of a placeholder than a request at this time. Support for AVIF gain maps is a much higher priority, with immediate practical applications and widespread support.

Current Canary support does not include gain maps (just SDR and simple HDR for JXL). Assuming JXL proceeds to mainline Chrome (and naturally to all related Chromium browsers), there would still be a very long tail for the upgrade cycle to have sufficient JXL support in the installed base (and FireFox does not support JXL today either). However, support in Canary is very encouraging and it would be ideal to support JXL gain maps down the road if the ecosystem support shows suitable progress to justify the effort.