layout: Add a first pass at incremental box tree construction (#37751)

This change:

- Adds a new type of LayoutDamage that signifies that a box needs its
  children recollected, because one or more of them need to be rebuilt.
- During restyle damage propagation, propagate this new damage upward in
  the tree. Then box tree construction should be able to preserve any
  still-valid box tree nodes from box slots.
- During BlockLevelBox job finalization, if a box slot is valid and
  there is not LayoutDamage to the element, use the old box slot,
  ensuring that its fragment cache is invalidated.

Testing: This should not change observable behavior and thus is covered
by existing WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>
Co-authored-by: coding-joedow <ibluegalaxy_taoj@163.com>

components/script/dom/node.rs

@@ -3927,6 +3927,9 @@ impl VirtualMethods for Node {
 pub(crate) enum NodeDamage {
     /// The node's `style` attribute changed.
     Style,
+    /// The node's content or heritage changed, such as the addition or removal of
+    /// children.
+    ContentOrHeritage,
     /// Other parts of a node changed; attributes, text content, etc.
     Other,
 }