[css-images-4] sRGB assumption in cross-fade

This issue was spun out of

• https://github.com/w3c/csswg-drafts/issues/9405

In CSS Images 4, 2.6.2. cross-fade Painting it states:

For the purpose of this calculation, each pixel’s color must be in pre-multiplied sRGB.

1. What to do with solid colors or pixel values specified in a non-sRGB colorspace is unspecified
2. The assumption is that the interpolation happens in (gamma-encoded) sRGB space, thus giving results that are too dark
3. There is no way to specify a different interpolation color space
4. The order of operations (when there are more than two colors or images in the cross-fade) is unspecified.

Steps to fix this:

1. Add a a <color-interpolation-method> token, as is already used in color-mix(), in palette-mix() and in linear, radial and conic gradients. This could be optional (with an sRGB default) or mandatory (since there are no implementations and thus no backwards compat risk)
2. Reference the order of operations from 12. Color Interpolation so that premultiplication, handling of missing components, etc is consistent with the rest of CSS. This would need to be extended to handle missing components when there are 3 or more inputs, which looks messy, which is why pairwise interpolation is used everywhere else.
3. Define what happens when non-sRGB colors get converted to sRGB (extended range transfer function, components can exceed the range [0,1], as specified in [10.2. The Predefined sRGB Color Space: the sRGB keyword](https://drafts.csswg.org/css-color-4/#predefined-sRGB))

Start with CSS Images 4 §2.6.2, then compare CSS Color 4 §12 and the color-interpolation-method usage in color-mix(), palette-mix(), and gradients. Resolve the interpolation-space, operation-order, missing-component, and non-sRGB conversion choices, then update the cross-fade specification so each case is defined.