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"

src/libslic3r/Layer.hpp

@@ -46,7 +46,7 @@ public:
 
     // collection of expolygons representing the bridged areas (thus not
     // needing support material)
-    Polygons                    bridged;
+//    Polygons                    bridged;
 
     // collection of polylines representing the unsupported bridge edges
     Polylines          			unsupported_bridge_edges;