Define "context" for SVG-in-OpenType glyphs

[AmeliaBR]

The SVG spec defines the "context element" for the context-fill and context-stroke keywords. However, it doesn't include the case of the context element for font glyphs, where the context element should be the element in the document that is using the font for text rendering. We probably assumed that the OpenType spec would define it, but it doesn't explicitly, instead referring back to SVG 2.

User agents that support SVG in OpenType fonts (Edge, Firefox, Illustrator) are all consistent about this behavior so it is stable enough to include in SVG 2.0. (Of course, we still need to get implementation commitments for using these keywords outside of fonts, but that's another issue.)

[AmeliaBR]

Related to #421

That issue is much more complicated, about which parts of the context should inherit into the font glyphs by default. This issue is simply about clearly defining that the context exists.

[svgeesus]

There is an attempt to define the context for font glyphs but it is in the wrong place, tacked onto the end of the font-specific UA stylesheet definition. That wording should be moved to the context element definition, and improved there.

[Tavmjong]

Inkscape supports context-fill and context-stroke outside of OpenType fonts. I submitted tests for this use recently.

[boggydigital]

Not blocking updated 2.0 CR publication - assigning 2.0 Recommendation milestone to clean this up before 2.0 REC