Auto merge of #13804 - canaltinova:gradient-parsing, r=Manishearth

Fix radial gradient's <size>/<ending-shape> parsing

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Parsing now handles sizes and shapes in various order.
I had to delete `EndingShape`'s parse implementation and mix it with `Position` in `parse_radial`. It became a bit complicated to read but I couldn't make it simpler.
r? @Manishearth

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13664 (github issue number if applicable).

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- [X] There are tests for these changes

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tests/unit/style/parsing/mod.rs

@@ -31,6 +31,27 @@ macro_rules! assert_roundtrip {
     }
 }
 
+macro_rules! assert_roundtrip_with_context {
+    ($fun:expr, $string:expr) => {
+        assert_roundtrip_with_context!($fun, $string, $string);
+    };
+    ($fun:expr,$input:expr, $output:expr) => {
+        let url = Url::parse("http://localhost").unwrap();
+        let context = ParserContext::new(Origin::Author, &url, Box::new(CSSErrorReporterTest));
+        let mut parser = Parser::new($input);
+        let parsed = $fun(&context, &mut parser)
+                     .expect(&format!("Failed to parse {}", $input));
+        let serialized = ::cssparser::ToCss::to_css_string(&parsed);
+        assert_eq!(serialized, $output);
+
+        let mut parser = Parser::new(&serialized);
+        let re_parsed = $fun(&context, &mut parser)
+                        .expect(&format!("Failed to parse {}", $input));
+        let re_serialized = ::cssparser::ToCss::to_css_string(&re_parsed);
+        assert_eq!(serialized, re_serialized);
+    }
+}
+
 
 macro_rules! parse_longhand {
     ($name:ident, $s:expr) => {{
@@ -41,6 +62,7 @@ macro_rules! parse_longhand {
 }
 
 mod basic_shape;
+mod image;
 mod mask;
 mod position;
 mod selectors;