When text paintables shift around in the tree due to line wrapping,
we may end up in a situation where some text node does not generate
a paintable (due to being all whitespace, for example), even though
in the previous layout pass, it *did* generate a paintable.
To prevent holding on to old paintables in such cases, we now do a
pass in LayoutState::commit() where we explicitly detach all old
paintables from the layout tree.
