The fast-layout engine now covers more of what busy pages do while loading

A follow-on to the last release's speed work on the built-in engine. When a page adds or removes content as it loads, or a change in one place pushes the things around it, the engine now redoes the layout for just the affected region instead of the whole page - including on table-based layouts, which many older and content-heavy sites still use. Two sizing glitches from the last release are fixed too, so an image or a flexible box measured while a page is still settling comes out the right size.