The paintable tree structure more closely matches the painting order
when fragments are owned by corresponding inline paintables. This
change does not affect the layout tree, as it is more convenient for
layout purposes to have all fragments owned by a block container in
one place.
Additionally, this improves performance significantly on pages with
many fragments, as we no longer have to walk the ancestor chain up
to the closest block container to determine if a fragment belongs
to an inline paintable.
With this change, a stacking context can be established by any
paintable, including inline paintables. The stacking context traversal
is updated to remove the assumption that the stacking context root is
paintable box.
Fragments contained by the inline node should be painted in the
foreground phase for this node, instead of being painted as a part of
the containing PaintableWithLines. This change implements that by
marking all fragments contained by inline nodes so they can be skipped
while painting the content of PaintableWithLines. This is an ugly way,
and instead, we should make InlinePaintables own all fragments
contained by them.
This fixes the issue that occurred when, after clicking an inline
paintable page would always scroll to the top. The problem was that
`scroll_an_element_into_view()` relies on `get_bounding_client_rect()`
to produce the correct scroll position and for inline paintables we
were always returning zero rect before this change.
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.
