Get rid of the separate renderContext stack and instead have a parent
pointer on renderer nodes. This lets us walk back up the render tree to
resolve context references
This mechanism will only be triggered by changes to the stylesheet
tiddlers themselves, and not to any dependencies that they might have.
It'll stay that way for a while.
The next thing is making the theme manager actually switch themes,
forcing the right wiki change events to make the stylesheet manager
update itself
