[css-color] Fallback color in @color-profile that uses RCS

There is the open question of what to do if a color profile fails to load, cannot be decoded, or simply while it's loading. https://github.com/w3c/csswg-drafts/issues/6129 contains the existing discussion on this.

Some implementors have expressed a reluctance to implement @color-profile because they do not want something as basic as colors to depend on an HTTP request. So, a good fallback strategy is essential for getting this rule implemented.

This is to propose another idea, which serves both as a fallback strategy, but also as a separate feature in its own right.

A mandatory fallback-color descriptor for @color-profile, which would accept any <color>, as well as support Relative Color Syntax with the components that the components descriptor has defined for the current color space.

In its most basic form, it can be a solid color, though that should be discouraged, for accessibility:

@color-profile --fancy {
	src: url('...');
	components: foo, bar, baz;
	fallback-color: red;
}

It can be used to define a fallback color space, e.g. in this case display-p3:

@color-profile --fancyrgb {
	src: url('...');
	components: r, g, b;
	fallback-color: color(display-p3 r g b / alpha);
}

But, most importantly, if there's no good fallback color space, it can be used to define a mapping to an existing space:

@color-profile --fancyrgb {
	src: url('...');
	components: r, g, b;
	fallback-color: color(display-p3 calc(r * .8) calc(g * .8) calc(b * .8));
}

@color-profile --betterlch {
	src: url('...');
	components: l, c, h;
	fallback: lch(calc(l / 100) calc(c / 300) h / alpha);
}

And, bonus, if we make the src descriptor optional, it can even be used to define custom color functions!

@color-profile --brand-red {
	components: l;
	fallback: lch(l 40 20 / alpha);
}

/*  use like color(--brand-red 40) or color(--brand-red 50 / .4) */

(in which case maybe we should rename it to something more generic like @color-space or simply @colors)

Start with the existing discussion in issue #6129 and the CSS Color 5 Relative Color Syntax section linked here. Compare the proposed mandatory fallback-color descriptor, its relative-color examples, and the optional-src/custom-function idea. This is complete when the CSSWG agrees on the fallback strategy, descriptor semantics, and any naming changes.