Removal of not numerically robust libraries "poly2tree" and "polypartition".

Adjustment of GUI/3DBed.cpp,hpp to use the more stable triangulation algoritm
derived from SGI glut.
Fix of an extremely slow bridging calculation, caused by an extremely
slow bridged area detection function, of which the results were never used.
Fixes "slicing fails or takes too long #5974"

doc/Dependencies.md

@@ -22,8 +22,6 @@
 * qhull: libqhull-dev does not contain libqhullcpp => link errors. Until it is fixed, we will use the builtin version. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=925540
 * semver: One module C library, author expects to use clib for installation. No packages.
 * Shiny: no packages
-* poly2tree: Obsolete, candidate for removal
-* polypartition: Obsolete, candidate for removal
 
 ## Header only
 * igl