[css-images-4] Add a parameter to image-set for HDR images

[chrishtr]

image-set has a <resolution> parameter which specifies the pixel density at which to prefer certain image resources. It'd be great to also include the HDR dimension, similar to the dynamic-range media query, to make it easier for developers to provide images optimized for HDR displays only when such a display is used.

[tabatkins]

Whatever we do here, we should coordinate with <img srcset>.

[lgarron]

This would be really, really useful for a Firefox bug that is going unaddressed: https://bugzilla.mozilla.org/show_bug.cgi?id=1793091

In theory, the "best of both worlds" for sites that want HDR images is to use a .avif file with an HDR color space. Safari and Chrome will apply very reasonable tone mapping. However, Firefox does not apply tone mapping, and the image shows up super dark:

I can think of ways to work around this, but they all involve either:

• Server side sniffing and SSR
• CSS hacks (as compared to declarative HTML)
• Client-side JavaScript to fix up the DOM based on user agent sniffing (or maaaybe HDR support sniffing, I haven't tested this because it's the worst form of progressive enhancement)

As there are more image formats coming down the pipeline, it would be really nice to have a way to do this using image-set instead of manually testing and sniffing for browser capabilities (and bugs, in some cases).