We sometimes had a stale stacking context tree sitting around, causing
incorrect paints until the next full layout invalidation.
Fix this by simply rebuilding the stacking context tree when asked to.
There's a subtle difference here. A "block box" in the spec is a
block-level box, while a "block container" is a box whose children are
either all inline-level boxes in an IFC, or all block-level boxes
participating in a BFC.
Notably, an "inline-block" box is a "block container" but not a "block
box" since it is itself inline-level.
