This implements the basic image-set notation without the format()
function (for simplicity).
There's a remaining serialization issue (we should probably skip 1x
resolutions), but that's fine for now, I'll address this in a follow-up
when the feature is testable.
The intention is to do the image selection at computed value time
(keeping a selected index or such), but same, follow-up.
This also fixes an issue where the cors-mode for -moz-image-rect and
cross-fade() was getting ignored when parsing.
Differential Revision: https://phabricator.services.mozilla.com/D100640
