As it turns out, Layout::TreeBuilder never managed to wrap text within
table boxes in anonymous wrapper boxes, since it relied on checking
text_for_rendering(), and that was never initialized during that early
stage of tree building.
This patch fixes the issue by making text_for_rendering() compute the
(potentially collapsed) text lazily when called.
Note that the test included with this patch is still totally wrong,
but that is now a TFC problem rather than a TreeBuilder problem. :^)
