Auto merge of #17202 - BorisChiou:stylo/animation/mismatched_transform, r=Manishearth,birtles

stylo: Bug 1335998 - Handle interpolation and accumulation of mismatched transform lists

These are the interdependent patches of Bug 1335998. We want to do interpolation and accumulation for mismatched transform lists, so introduce ComputedOperation::InterpolateMatrix and ComputedOperation::Accumulation. Both arms store the from_list and to_list, and resolve them until we have the layout information. For the Servo part, we haven't implemented how to read the transform lists in layout/fragment.rs, but I think it would be easy. (related issue #13267)

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1335998](https://bugzilla.mozilla.org/show_bug.cgi?id=1335998)
- [X] There are tests for these changes

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components/layout/fragment.rs

@@ -2896,6 +2896,12 @@ impl Fragment {
                     Matrix4D::create_skew(Radians::new(theta_x.radians()),
                                           Radians::new(theta_y.radians()))
                 }
+                transform::ComputedOperation::InterpolateMatrix { .. } |
+                transform::ComputedOperation::AccumulateMatrix { .. } => {
+                    // TODO: Convert InterpolateMatrix/AccmulateMatrix into a valid Matrix4D by
+                    // the reference box.
+                    Matrix4D::identity()
+                }
             };
 
             transform = transform.pre_mul(&matrix);