This simplifies the ownership model between DOM/layout/paint nodes
immensely by deferring to the garbage collector for figuring out what's
live and what's not.
This patch adds a bunch of Paintable subclasses, each corresponding to
the Layout::Node subclasses that had a paint() override. All painting
logic is moved from layout nodes into their corresponding paintables.
Paintables are now created by asking a Layout::Box to produce one:
static NonnullOwnPtr<Paintable> Layout::Box::create_paintable()
Note that inline nodes still have their painting logic. Since they
are not boxes, and all paintables have a corresponding box, we'll need
to come up with some other solution for them.
The "paintable" state in Layout::Box was actually not safe to access
until after layout had been performed.
As a first step towards making this harder to mess up accidentally,
this patch moves painting information from Layout::Box to a new class:
Painting::Box. Every layout can have a corresponding paint box, and
it holds the final used metrics determined by layout.
The paint box is created and populated by FormattingState::commit().
I've also added DOM::Node::paint_box() as a convenient way to access
the paint box (if available) of a given DOM node.
Going forward, I believe this will allow us to better separate data
that belongs to layout vs painting, and also open up opportunities
for naturally invalidating caches in the paint box (since it's
reconstituted by every layout.)
