ENH: improve supporting sharp tails of tree support

1. sharp tails are supported by a sparse set of contact points which are
easier to remove than previously dense surrounding support.
   Organic tree support also has this feature, including all other smart
overhang detection techniques (small overhang and cantilever detection),
with the cost of slightly longer time to detect overhangs.
2. improve supporting overhang contours by adding contact points along
   contours.
  jira: STUDIO-3876
2. remove some redundant data structure.

Change-Id: If7f595348506a14aba2d0132d23f97d3539c1e1f
(cherry picked from commit e3cce09b9db12ced2841045ffd337b1f35494e6c)
(cherry picked from commit 507345deb193d895d0813fc913f00b0def7e62f9)

src/libslic3r/Layer.hpp

@@ -275,11 +275,10 @@ public:
     ExPolygons                  support_islands;
     // Extrusion paths for the support base and for the support interface and contacts.
     ExtrusionEntityCollection   support_fills;
-    SupportInnerType          support_type = stInnerNormal;
+    SupportInnerType            support_type = stInnerNormal;
 
     // for tree supports
     ExPolygons base_areas;
-    ExPolygons                                overhang_areas;
 
 
     // Is there any valid extrusion assigned to this LayerRegion?
@@ -318,9 +317,7 @@ protected:
         bool need_extra_wall = false;
         AreaGroup(ExPolygon *a, int t, coordf_t d) : area(a), type(t), dist_to_top(d) {}
     };
-    enum OverhangType { Detected = 0, Enforced, SharpTail };
     std::vector<AreaGroup>                    area_groups;
-    std::map<const ExPolygon *, OverhangType> overhang_types;
 };
 
 template<typename LayerContainer>