Refactored Point / MultiPoint / Polyline / Polygon:

1) Removed virtual methods. There was not really need for them.
2) Some of the virtual methods were using conversion to Lines, which
   was unnecessary and expensive.
3) Removed some nearest element search methods from Point.

src/libslic3r/GCode.cpp

@@ -116,8 +116,7 @@ namespace Slic3r {
             Point pos = Point::new_scale(writer_pos(0), writer_pos(1));
 
             // find standby point
-            Point standby_point;
-            pos.nearest_point(this->standby_points, &standby_point);
+            Point standby_point = nearest_point(this->standby_points, pos).first;
 
             /*  We don't call gcodegen.travel_to() because we don't need retraction (it was already
                 triggered by the caller) nor avoid_crossing_perimeters and also because the coordinates