layout: separate out tree construction logic

For now, the skeleton remains in layout.  Eventually it should be
lazily constructed during the actual layout pass (thereby making layout
"single-pass" (sometimes :p))

The end goal is to do it all in styledNode.children, so that caching can
be as simple as "next box = find next matching cached box ?? make new".

This does mean that the internal structure of cached boxes will always
have to be reconstructed, but I don't see a better way.  (I suppose it
still remains possible to optimize out the unnecessary layout pass when
only a repaint is needed (e.g. color change) by modifying computed
values in-place.)

1 files changed, 1 insertions, 0 deletions

diff --git a/test/layout/pseudo-element-display-table-empty-content.expected b/test/layout/pseudo-element-display-table-empty-content.expected
new file mode 100644
index 00000000..8b137891
--- /dev/null
+++ b/test/layout/pseudo-element-display-table-empty-content.expected
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+