`BoxFragment` (indirectly) owns an `Arc<AtomicRefCell<ComputedStyle>>`.
To access, `&ComputedStyle`, the `AtomicRefCell` must increment an atomic
borrow counter and return a guard that decrements the counter when dropped.
Layout code accesses the style in may places, but repeatdly borrowing and
unborrowing the refcell creates additional traffic of the atomic borrow counter
which may have some performance cost.
The new `BoxFragmentWithStyle<'_>` struct packages a `&BoxFragment` with an
already-borrowed guard for the style, so using it to repeatedly access the style
is free.
To start with, this is used in `display_list::BuilderForBoxFragment`.
We can later extend its use in more places that repeatedly access style.
Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

+ drive-by remove dubious `#[inline]`
Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Signed-off-by: Simon Sapin <simon@igalia.com>

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Signed-off-by: Simon Sapin <simon@igalia.com>