Define ComputedOperation::InterpolateMatrix.

We use this arm to store the interpolated result of two mismatched
transform lists, and we resolve it until we know the reference box size
(on Gecko side). The conversion from ComputedOperation::InterpolateMatrix
to eCSSKeyword_interpolatematrix will be implemented later in this patch series.

components/layout/fragment.rs

@@ -2896,6 +2896,10 @@ impl Fragment {
                     Matrix4D::create_skew(Radians::new(theta_x.radians()),
                                           Radians::new(theta_y.radians()))
                 }
+                transform::ComputedOperation::InterpolateMatrix { .. } => {
+                    // TODO: Convert InterpolateMatrix into a valid Matrix4D by the reference box.
+                    Matrix4D::identity()
+                }
             };
 
             transform = transform.pre_mul(&matrix);