Fix the identity transform of rotatex and rotatey.

Let's see the example: "rotatex(360deg)" to "none".

While we do interpolation between "rotatex" and "none", the original code
path is:
1. Build an identity transform for none and always use (0, 0, 1) as the
   direction vector, i.e. Rotate(0, 0, 1, 0deg).
2. Do interpolation between rotatex (i.e. Rotate(1, 0, 0, 360deg)) and
   Rotate(0, 0, 1, 0deg). Because their direction vectors are different,
   so we do matrix decomposition/interpolation/recomposition on both
   functions. The problem is, matrix decomposition makes the 360deg
   disappear, so it looks like "rotatex(0deg)".
3. Obviously, we are trying to interpolate from rotatex(0deg) to none, so
   the interpolated matrix is always an identity matrix.

I think rotatex, rotatey, and rotatez are special cases which should
really rotate according to the angle, so we should build the identity
transform for them according to the normalized direction vector, and so we do
interpolation on the angle, instead of converting them into matrix.

Replacing build_identity_transform_list() with
add_weighted_transform_lists(list, list, 0, 0) might be another solution;
However, I didn't do that because build_identity_transform_list() is
much simpler.

components/style/properties/helpers/animated_properties.mako.rs

@@ -1340,8 +1340,9 @@ fn build_identity_transform_list(list: &[TransformOperation]) -> Vec<TransformOp
             TransformOperation::Scale(..) => {
                 result.push(TransformOperation::Scale(1.0, 1.0, 1.0));
             }
-            TransformOperation::Rotate(..) => {
-                result.push(TransformOperation::Rotate(0.0, 0.0, 1.0, Angle::zero()));
+            TransformOperation::Rotate(x, y, z, a) => {
+                let (x, y, z, _) = get_normalized_vector_and_angle(x, y, z, a);
+                result.push(TransformOperation::Rotate(x, y, z, Angle::zero()));
             }
             TransformOperation::Perspective(..) |
             TransformOperation::AccumulateMatrix { .. } |