style: Implement parsing and serialization for most of image-set().

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

components/style/values/specified/resolution.rs

@@ -12,7 +12,7 @@ use cssparser::{Parser, Token};
 use style_traits::{ParseError, StyleParseErrorKind};
 
 /// A specified resolution.
-#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss, ToShmem)]
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, ToCss, ToShmem, SpecifiedValueInfo)]
 pub enum Resolution {
     /// Dots per inch.
     #[css(dimension)]