The patch simply makes fo:wrapper add LMs for all its children when
its parent requests it to add its own LM. Disadvantage: the pattern of
incremental addition of childLMs is broken; the childLMs for all
children of fo:wrapper are added at once.

In a patch that would enable fo:wrapper to return its childLMs one by
one, the parent's childLMiter.preLoadNext() would have to ask a child
for a childLM more than once; therefore it would have to ask the
current child first (baseIter.previous()) if it wants to contribute
more LMs, before moving on to the next child.