Auto merge of #12839 - notriddle:11818_sequential_layout_bug, r=emilio

Fix a cached style cascade bug that only manifested in sequential mode

When copying cached styles, keep the `writing_mode` up to date.

---

- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11818 (github issue number if applicable).
- [X] There are tests for these changes

EDIT: The test is now working. I ran it with the first commit (the actual fix) reverted and it failed.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12839)
<!-- Reviewable:end -->

components/style/properties/properties.mako.rs

@@ -1716,6 +1716,9 @@ fn cascade_with_cached_declarations(
         context.mutate_style().mutate_font().compute_font_hash();
     }
 
+    let mode = get_writing_mode(context.style.get_inheritedbox());
+    context.style.set_writing_mode(mode);
+
     context.style
 }