ENH: tree support style default to Tree Hybrid

1. tree support style default to Tree Hybrid
2. fix an assert bug with MedialAxis::build:
    assert(polyline.width.size() == polyline.points.size() * 2 - 2)
  This is caused by the incorrect clear function of ThickPolyline.

Change-Id: I8d6507ad366d253493e2749e0cbac8d3d837ee8a
(cherry picked from commit bd31e962c11e82005944bef40b17471075e79898)

src/libslic3r/TreeSupport.hpp

@@ -381,6 +381,7 @@ public:
     bool  has_sharp_tails = false;
     bool  has_cantilever = false;
     SupportType support_type;
+    SupportMaterialStyle support_style;
 
     std::unique_ptr<FillLightning::Generator> generator;
     std::unordered_map<double, size_t> printZ_to_lightninglayer;