Add properties of Fill and Stroke spec plus `paint-order` to properties that apply to `::cue` and `::cue()`

[SebastianZ]

As noted in a Mozilla bug, the WebVTT spec should probably add the properties defined in the Fill and Stroke spec plus the paint-order property from SVG 2 to the properties allowed in the ::cue and ::cue() pseudo-elements.

Maybe also the prefixed variants of the standardized Fill and Stroke properties -webkit-text-stroke, -webkit-text-stroke-width, and -webkit-text-stroke-color for compatibility.
Of course, there's no compatibility issue for WebVTT itself, as it doesn't support the standardized variants yet. Though at the moment, browsers only support the prefixed variants in HTML context and it would be inconsistent to have them apply in HTML but not WebVTT.

Sebastian

[gkatsev]

That probably makes sense to me.

Probably would specify something like we do for text-decoration

webvtt/index.bs

Line 5340 in 9966609

<li>the properties corresponding to the 'text-decoration' shorthand</li>

the properties corresponding to the [stroke](https://drafts.csswg.org/fill-stroke/#propdef-stroke) shorthand
the properties corresponding to the [fill](https://drafts.csswg.org/fill-stroke/#propdef-fill) shorthand

@eric-carlson @jernoble thoughts on this? Any concerns?