style: Make SVGPathData and clip-path: path() animatable.

Implement Animate trait for SVGPathData.

The basic idea is: we normalize |this| and |other| svg paths, and then
do interpolation on the normalized svg paths. The normalization is to
convert relative coordinates into absolute coordinates, so we could do
real number interpolation on each path command directly.

In this patch, we also make |clip-path:path()| animatable.

Differential Revision: https://phabricator.services.mozilla.com/D4786

components/style/values/animated/mod.rs

@@ -141,6 +141,20 @@ impl Animate for f64 {
     }
 }
 
+/// This is only used in SVG PATH. We return Err(()) if the flags are mismatched.
+// FIXME: Bug 653928: If we want to do interpolation on the flags in Arc, we have to update this
+// because `absolute`, `large_arc_flag`, and `sweep_flag` are using this implementation for now.
+impl Animate for bool {
+    #[inline]
+    fn animate(&self, other: &Self, _procedure: Procedure) -> Result<Self, ()> {
+        if *self == *other {
+            Ok(*other)
+        } else {
+            Err(())
+        }
+    }
+}
+
 impl<T> Animate for Option<T>
 where
     T: Animate,