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/gecko/url.rs

@@ -285,14 +285,15 @@ impl SpecifiedImageUrl {
 
     /// Provides an alternate method for parsing that associates the URL
     /// with anonymous CORS headers.
-    pub fn parse_with_cors_anonymous<'i, 't>(
+    pub fn parse_with_cors_mode<'i, 't>(
         context: &ParserContext,
         input: &mut Parser<'i, 't>,
+        cors_mode: CorsMode,
     ) -> Result<Self, ParseError<'i>> {
         Ok(SpecifiedImageUrl(SpecifiedUrl::parse_with_cors_mode(
             context,
             input,
-            CorsMode::Anonymous,
+            cors_mode,
         )?))
     }
 }