style: Use cbindgen for filters.

Had to implement some OwnedSlice bits that the canvas code used.

Differential Revision: https://phabricator.services.mozilla.com/D31799

components/style/values/generics/effects.rs

@@ -34,8 +34,10 @@ pub struct GenericBoxShadow<Color, SizeLength, BlurShapeLength, ShapeLength> {
 pub use self::GenericBoxShadow as BoxShadow;
 
 /// A generic value for a single `filter`.
+///
+/// cbindgen:derive-tagged-enum-copy-constructor=true
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
-#[animation(no_bound(Url))]
+#[animation(no_bound(U))]
 #[derive(
     Clone,
     ComputeSquaredDistance,
@@ -49,7 +51,8 @@ pub use self::GenericBoxShadow as BoxShadow;
     ToResolvedValue,
     ToShmem,
 )]
-pub enum Filter<Angle, Factor, Length, DropShadow, Url> {
+#[repr(C, u8)]
+pub enum GenericFilter<Angle, Factor, Length, Shadow, U> {
     /// `blur(<length>)`
     #[css(function)]
     Blur(Length),
@@ -79,12 +82,14 @@ pub enum Filter<Angle, Factor, Length, DropShadow, Url> {
     Sepia(Factor),
     /// `drop-shadow(...)`
     #[css(function)]
-    DropShadow(DropShadow),
+    DropShadow(Shadow),
     /// `<url>`
     #[animation(error)]
-    Url(Url),
+    Url(U),
 }
 
+pub use self::GenericFilter as Filter;
+
 /// A generic value for the `drop-shadow()` filter and the `text-shadow` property.
 ///
 /// Contrary to the canonical order from the spec, the color is serialised