According to https://eigen.tuxfamily.org/dox/TopicPitfalls.html one
should just avoid using `auto` as the type of an Eigen expression.
This PR fixes most of them I could found in the project. There might be
cases that I missed, and I might update those later if I noticed.
This should prevent issues like #7741 and hopefully fix some mysterious
crashes happened inside Eigen calls.
* Pass all compatible regions to perimeter generator
* Simplify & unify fuzzify detection
* Simplify `to_thick_polyline`
* Group regions by fuzzy skin settings
* Initial code structure of multi-regional fuzzy skin
* Determine fuzzy type by all compatible regions
* Add fuzzy region debug
* Implement the line split algorithm
* Do splitted fuzzy in classic mode
* Disable debug macros
* Fix infinit loop issue when segment points are out of order
* Fix path connection
* Implement splitted fuzzy in Arachne mode