Make use of generics for transform-origin

This allows us to preserve keywords during serialisation.

components/style/values/computed/transform.rs

@@ -0,0 +1,49 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Computed types for CSS values that are related to transformations.
+
+use properties::animated_properties::Animatable;
+use values::computed::{Length, LengthOrPercentage};
+use values::generics::transform::TransformOrigin as GenericTransformOrigin;
+
+/// The computed value of a CSS `<transform-origin>`
+pub type TransformOrigin = GenericTransformOrigin<LengthOrPercentage, LengthOrPercentage, Length>;
+
+impl TransformOrigin {
+    /// Returns the initial computed value for `transform-origin`.
+    #[inline]
+    pub fn initial_value() -> Self {
+        Self::new(
+            LengthOrPercentage::Percentage(0.5),
+            LengthOrPercentage::Percentage(0.5),
+            Length::from_px(0)
+        )
+    }
+}
+
+impl Animatable for TransformOrigin {
+    #[inline]
+    fn add_weighted(&self, other: &Self, self_portion: f64, other_portion: f64) -> Result<Self, ()> {
+        Ok(Self::new(
+            self.horizontal.add_weighted(&other.horizontal, self_portion, other_portion)?,
+            self.vertical.add_weighted(&other.vertical, self_portion, other_portion)?,
+            self.depth.add_weighted(&other.depth, self_portion, other_portion)?,
+        ))
+    }
+
+    #[inline]
+    fn compute_distance(&self, other: &Self) -> Result<f64, ()> {
+        self.compute_squared_distance(other).map(f64::sqrt)
+    }
+
+    #[inline]
+    fn compute_squared_distance(&self, other: &Self) -> Result<f64, ()> {
+        Ok(
+            self.horizontal.compute_squared_distance(&other.horizontal)? +
+            self.vertical.compute_squared_distance(&other.vertical)? +
+            self.depth.compute_squared_distance(&other.depth)?
+        )
+    }
+}