Intent to ship: enhanced color font (COLRv1) support

[Jonathan Kew]

As of Firefox 107, I intend to turn COLRv1 font support on by default.
It has been developed behind the gfx.font_rendering.colr_v1.enabled
preference.

Status in other browsers:

- Chrome (and other Blink-based browsers, e.g. Edge): partial
implementation currently shipping; full impl currently behind a pref in
Canary, hoping to ship in Chrome 107 (mid-October).

- Safari: no current plans, AFAIK.

Bug to turn on by default:

https://bugzilla.mozilla.org/show_bug.cgi?id=1791558

Standard:

- https://docs.microsoft.com/en-gb/typography/opentype/spec/colr
- https://github.com/googlefonts/colr-gradients-spec

This feature was previously discussed in this "Intent to prototype"
thread: https://groups.google.com/a/mozilla.org/g/dev-platform/c/u7gjLLRkfy0

- JK