layout: Measure stacking context tree in layout thread memory report (#38762)

Measures the memory usage of the stacking context tree in the memory
report of the layout thread by adding `MallocSizeOf` to
`StackingContextTree` and all the types required for that. Also requires
adding `MallocSizeOf` to some webrender types.

Testing: Manually looked at about:memory
<img width="636" height="241" alt="image"
src="https://github.com/user-attachments/assets/6bf9d65a-0bf0-4a99-99b5-ddedba3269c1"
/>

Fixes: https://github.com/servo/servo/issues/38725

---------

Signed-off-by: Rahul Menon <menonrahul02@gmail.com>

components/layout/layout_impl.rs

@@ -459,6 +459,12 @@ impl Layout for LayoutThread {
                 .unwrap_or_default(),
         });
 
+        reports.push(Report {
+            path: path![formatted_url, "layout-thread", "stacking-context-tree"],
+            kind: ReportKind::ExplicitJemallocHeapSize,
+            size: self.stacking_context_tree.size_of(ops),
+        });
+
         reports.push(self.image_cache.memory_report(formatted_url, ops));
     }