It was a right cow to get working at all due to the tangled mess of
dependencies between different hierarchies (switching to hierarchies as
components helpt), but other that some vertical positioning (paragraphs
and descenders), it's working fairly well now (and fairly quick other
than I think I need to ensure the shaping cache is used).
This is for scroll boxes (the nesting of canvases is for the clipping
they provide). There are some issues with automatic layout, but this
gets things mostly working, in particular the management of the link
between hierarchies as a canvas is always the root of its hierarchy.
