Fix infinite loop in neatogen multispline

There were two problems that caused the non-termination in #2168 (closed):

• One very old that existed prior to b1a03d58 and affected all node shapes.
• One that was introduced by b1a03d58 and affected ellipse shaped nodes with penwidth ⪆ 1.

Several other related problems was discovered while investigating this. For details, see the commit messages.