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.)
This patch begins the support for the 'view-box' attribute that can be
attached to <svg>'s.
The FormattingContext determines the size of the Element according to
the specified 'width' and 'height' or if they are not given by the
'viewbox' or by the bounding box of the path if nothing is specified.
When we try to paint a SVG Path that belongs to a <svg> that has the
'view-box' and a specified 'height'/'width', all the parts of the path
get scaled/moved accordingly.
There probably are many edge cases and bugs still to be found, but this
is a nice start. :^)
This makes the selected-in-the-inspector outline appear in the right
place. We take the stroke-width into account when producing the
bounding box, which makes the fit nice and snug. :^)
