(the following is the original commit message)
Now that the layer permutation is finalized, it's possible to select
a couple of node-groups that only depend on a single predecessor and
consider them a single linear segment. This allows to "draw" straight
vertical lines accross the layout for nodes that could benefit from
that.
Such linear segments are also paramount for good looking long edges
(both forwards and backwards facing).
\note: this part of the layouter could still use more work.
If you're looking to rework it, this is the spot.
Ivan Krysak