By saving string with alt text, image paintable no longer need to reach
into layout and DOM nodes while painting commands recording.
No behaviour change intended.
With this change, elements that want to receive viewport rect updates
will need to register on document instead of the browsing context.
This change solves the problem where a browsing context for a document
is guaranteed to exist only while the document is active so browsing
context might not exit by the time DOM node that want to register is
constructed.
This is a part of preparation work before switching to navigables where
this issue becomes more visible.
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.
