[filter-effects] chained filter support, aka href on the filter element

[karlcow]

There was an issue on w3c/fxtf-drafts#383 about this.

We discussed about it on the SVG WG today
w3c/svgwg#1111

RESOLUTION: Suggestion to CSS WG: Remove possibility for filters to reference other filters. Remove the SVG URI DOM interface as well.

The current editors are @dirkschulze and @chrishtr

Clone of the issue:

<svg xmlns="http://www.w3.org/2000/svg">
    <filter id="filter">
        <feFlood flood-color="green"/>
        <feComposite in2="SourceAlpha" operator="atop"/>
    </filter>
    <filter id="chained" href="#filter"/>
    <rect x="10" y="10" width="100" height="100" filter="url(#filter)"/>
    <rect x="120" y="10" width="100" height="100" filter="url(#chained)"/>
</svg>

The filter spec is now at
https://drafts.csswg.org/filter-effects-1/

As mentioned in https://w3c.github.io/svgwg/svg2-draft/single-page.html#changes-filters

The filter element
https://drafts.csswg.org/filter-effects-1/#FilterElement
href is not defined as an attribute on this element.

the DOM interface indicates
https://drafts.csswg.org/filter-effects-1/#InterfaceSVGFilterElement

SVGFilterElement includes SVGURIReference;

which is itself:

interface mixin SVGURIReference {
  [SameObject] readonly attribute SVGAnimatedString href;
};

https://w3c.github.io/svgwg/svg2-draft/types.html#InterfaceSVGURIReference

[tabatkins]

Sounds fine to me.