Move the layout tree from GC allocation to refcounted ownership so
removed layout and paint subtrees are destroyed synchronously instead
of waiting for the next GC sweep. This dramatically reduces GC memory
usage peaks after layout tree churn and makes it easier for memory use
to fall back after large document updates.
Update layout factories, tree traversal, SVG layout node creation,
paintable back-pointers, and pseudo-element layout links to use RefPtr
ownership.
Make display: contents follow the same shape as Blink and WebKit: the
element itself does not create a layout node, and its children are
flattened into the nearest layout parent. Wrap direct non-whitespace
text in an anonymous inline node when the boxless element contributes
inherited style to that text.
Use an internal inline wrapper for display: contents pseudo-elements
so generated content can still participate in layout, painting, hit
testing, and pseudo-element queries. Keep CSSOM reporting the computed
display value from the pseudo style, not the internal wrapper.
Remove the retained out-of-tree layout node list and its testing hook,
since the flattened model does not need a side owner for boxless
elements. Add coverage for inherited text style, dynamic insertion
order, pseudo-element hit testing, and computed style queries.
We now apply first letter styles by splitting text with a first-letter
style applied into 2 `TextSliceNode` objects. The
`DOM::Text` layout node always points at the non first-letter slice
and the first-letter slice is reachable via
`TextSliceNode::first_letter_slice()`.
First letter splitting works by `TreeBuilder` walking a block
container's inline descendants to find the first typographic letter
unit per the pattern given in css-pseudo level 4, which is then
wrapped in an anonymous inline box styled with the `::first-letter`
computed properties.
Consumers that map between DOM offsets and layout geometry
are updated to visit all slices of a `DOM::Text` through
`TextOffsetMapping`.
