layout: Share styles to inline box children via SharedInlineStyles (#36896)

`TextRun`s use their parent style to render. Previously, these styles
were cloned and stored directly in the box tree `TextRun` and resulting
`TextFragment`s. This presents a problem for incremental layout.
Wrapping the style in another layer of shared ownership and mutability
will allow updating all `TextFragment`s during repaint-only incremental
layout by simply updating the box tree styles of the original text
parents.

This adds a new set of borrows when accessing text styles, but also
makes it so that during box tree block construction
`InlineFormattingContext`s are created lazily and now
`InlineFormattingContextBuilder::finish` consumes the builder, making
the API make a bit more sense. This should also improve performance of
box tree block construction slightly.

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>

components/layout/flow/root.rs

@@ -174,7 +174,7 @@ impl BoxTree {
 
             let update_point =
                 match &*AtomicRef::filter_map(layout_data.self_box.borrow(), Option::as_ref)? {
-                    LayoutBox::DisplayContents => return None,
+                    LayoutBox::DisplayContents(..) => return None,
                     LayoutBox::BlockLevel(block_level_box) => match &*block_level_box.borrow() {
                         BlockLevelBox::OutOfFlowAbsolutelyPositionedBox(_)
                             if box_style.position.is_absolutely_positioned() =>