Fixed ThickPolyline clear(), which did not clear widths.

Fixed by refactoring ThickPolyline to compose Polyline instead
of being derived of it.

src/libslic3r/Arachne/utils/ExtrusionLine.hpp

@@ -14,7 +14,7 @@
 #include "../../../clipper/clipper_z.hpp"
 
 namespace Slic3r {
-class ThickPolyline;
+struct ThickPolyline;
 }
 
 namespace Slic3r::Arachne

src/libslic3r/ExPolygon.cpp

@@ -320,7 +320,9 @@ void ExPolygon::medial_axis(double min_width, double max_width, Polylines* polyl
 {
     ThickPolylines tp;
     this->medial_axis(min_width, max_width, &tp);
-    polylines->insert(polylines->end(), tp.begin(), tp.end());
+    polylines->reserve(polylines->size() + tp.size());
+    for (auto &pl : tp)
+        polylines->emplace_back(pl.points);
 }
 
 Lines ExPolygon::lines() const

src/libslic3r/Geometry/MedialAxis.cpp

@@ -498,6 +498,7 @@ void MedialAxis::build(ThickPolylines* polylines)
             polyline.width.emplace_back(seed_edge_data.width_end);        
             // Grow the polyline in a forward direction.
             this->process_edge_neighbors(&*seed_edge, &polyline);
+            assert(polyline.width.size() == polyline.points.size() * 2 - 2);
         
             // Grow the polyline in a backward direction.
             reverse_polyline.clear();
@@ -505,7 +506,6 @@ void MedialAxis::build(ThickPolylines* polylines)
             polyline.points.insert(polyline.points.begin(), reverse_polyline.points.rbegin(), reverse_polyline.points.rend());
             polyline.width.insert(polyline.width.begin(), reverse_polyline.width.rbegin(), reverse_polyline.width.rend());
             polyline.endpoints.first = reverse_polyline.endpoints.second;
-        
             assert(polyline.width.size() == polyline.points.size() * 2 - 2);
         
             // Prevent loop endpoints from being extended.
@@ -538,7 +538,9 @@ void MedialAxis::build(Polylines* polylines)
 {
     ThickPolylines tp;
     this->build(&tp);
-    polylines->insert(polylines->end(), tp.begin(), tp.end());
+    polylines->reserve(polylines->size() + tp.size());
+    for (auto &pl : tp)
+        polylines->emplace_back(pl.points);
 }
 
 void MedialAxis::process_edge_neighbors(const VD::edge_type *edge, ThickPolyline* polyline)

src/libslic3r/Polyline.hpp

@@ -10,7 +10,7 @@
 namespace Slic3r {
 
 class Polyline;
-class ThickPolyline;
+struct ThickPolyline;
 typedef std::vector<Polyline> Polylines;
 typedef std::vector<ThickPolyline> ThickPolylines;
 
@@ -160,20 +160,28 @@ bool remove_degenerate(Polylines &polylines);
 // Returns index of a segment of a polyline and foot point of pt on polyline.
 std::pair<int, Point> foot_pt(const Points &polyline, const Point &pt);
 
-class ThickPolyline : public Polyline {
-public:
-    ThickPolyline() : endpoints(std::make_pair(false, false)) {}
+struct ThickPolyline {
+    ThickPolyline() = default;
     ThickLines thicklines() const;
+
+    const Point& first_point()  const { return this->points.front(); }
+    const Point& last_point()   const { return this->points.back(); }
+    bool         is_valid()     const { return this->points.size() >= 2; }
+    double       length()       const { return Slic3r::length(this->points); }
+
+    void         clear() { this->points.clear(); this->width.clear(); }
+
     void reverse() {
-        Polyline::reverse();
+        std::reverse(this->points.begin(), this->points.end());
         std::reverse(this->width.begin(), this->width.end());
         std::swap(this->endpoints.first, this->endpoints.second);
     }
 
     void clip_end(double distance);
 
+    Points                  points;
     std::vector<coordf_t>   width;
-    std::pair<bool,bool>  endpoints;
+    std::pair<bool,bool>    endpoints { false, false };
 };
 
 inline ThickPolylines to_thick_polylines(Polylines &&polylines, const coordf_t width)

src/libslic3r/SVG.cpp

@@ -179,8 +179,8 @@ void SVG::draw(const ThickLines &thicklines, const std::string &fill, const std:
 
 void SVG::draw(const ThickPolylines &polylines, const std::string &stroke, coordf_t stroke_width)
 {
-    for (ThickPolylines::const_iterator it = polylines.begin(); it != polylines.end(); ++it)
-        this->draw((Polyline)*it, stroke, stroke_width);
+    for (const ThickPolyline &pl : polylines)
+        this->draw(Polyline(pl.points), stroke, stroke_width);
 }
 
 void SVG::draw(const ThickPolylines &thickpolylines, const std::string &fill, const std::string &stroke, coordf_t stroke_width)