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/BridgeDetector.hpp

@@ -32,6 +32,7 @@ public:
     BridgeDetector(const ExPolygons &_expolygons, const ExPolygons &_lower_slices, coord_t _extrusion_width);
     // If bridge_direction_override != 0, then the angle is used instead of auto-detect.
     bool detect_angle(double bridge_direction_override = 0.);
+    // Coverage is currently only used by the unit tests. It is extremely slow and unreliable!
     Polygons coverage(double angle = -1) const;
     void unsupported_edges(double angle, Polylines* unsupported) const;
     Polylines unsupported_edges(double angle = -1) const;