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"

xs/xsp/ExPolygon.xsp

@@ -31,10 +31,6 @@
     Polygons simplify_p(double tolerance);
     Polylines medial_axis(double max_width, double min_width)
         %code{% THIS->medial_axis(max_width, min_width, &RETVAL); %};
-    Polygons get_trapezoids2(double angle)
-        %code{% THIS->get_trapezoids2(&RETVAL, angle); %};
-    Polygons triangulate()
-        %code{% THIS->triangulate(&RETVAL); %};
 %{
 
 ExPolygon*

xs/xsp/Layer.xsp

@@ -18,8 +18,6 @@
         %code%{ RETVAL = &THIS->thin_fills; %};
     Ref<SurfaceCollection> fill_surfaces()
         %code%{ RETVAL = &THIS->fill_surfaces; %};
-    Polygons bridged()
-        %code%{ RETVAL = THIS->bridged; %};
     Ref<ExtrusionEntityCollection> perimeters()
         %code%{ RETVAL = &THIS->perimeters; %};
     Ref<ExtrusionEntityCollection> fills()