Implement to_transform_3d_matrix for computing distance of mismatched transform lists.

We could use this method to convert a TransformList into a Matrix, and
use this matrix for computing distance for Stylo and rendering the transform
for Servo.

This is an equivalent of nsStyleTransformMatrix::ReadTransforms in Gecko.

components/style/values/computed/angle.rs

@@ -4,6 +4,7 @@
 
 //! Computed angles.
 
+use euclid::Radians;
 use std::{f32, f64, fmt};
 use std::f64::consts::PI;
 use style_traits::ToCss;
@@ -112,3 +113,10 @@ impl ToCss for Angle {
         }
     }
 }
+
+impl From<Angle> for Radians<CSSFloat> {
+    #[inline]
+    fn from(a: Angle) -> Self {
+        Radians::new(a.radians())
+    }
+}