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/slic3r/GUI/3DBed.hpp

@@ -17,20 +17,14 @@ class GeometryBuffer
 {
     struct Vertex
     {
-        float position[3];
-        float tex_coords[2];
-
-        Vertex()
-        {
-            position[0] = 0.0f; position[1] = 0.0f; position[2] = 0.0f;
-            tex_coords[0] = 0.0f; tex_coords[1] = 0.0f;
-        }
+        Vec3f position = Vec3f::Zero();
+        Vec2f tex_coords = Vec2f::Zero();
     };
 
     std::vector<Vertex> m_vertices;
 
 public:
-    bool set_from_triangles(const Polygons& triangles, float z, bool generate_tex_coords);
+    bool set_from_triangles(const std::vector<Vec2f> &triangles, float z);
     bool set_from_lines(const Lines& lines, float z);
 
     const float* get_vertices_data() const;