This replaces the usage of `rounded_int_rect`, whose name did not
accurately reflect the rounding operation happening. For example, the
position of the rect was not rounded but floored, and the size was
pulled through `roundf` before casting to `int` which could result in
inadvertent flooring if the resulting floating point could not exactly
represent the rounded value.
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.
