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/Polyline.hpp

@@ -234,6 +234,10 @@ public:
         std::reverse(this->width.begin(), this->width.end());
         std::swap(this->endpoints.first, this->endpoints.second);
     }
+    void clear() {
+        Polyline::clear();
+        width.clear();
+    }
 
     std::vector<coordf_t> width;
     std::pair<bool,bool>  endpoints;