FIX: fix some bugs in wipe_tower's skip points

jira:none

Change-Id: I387a168ccfe65032b3b926d02d8da4ab7aed317e
(cherry picked from commit cbcee1b79876b14a04e069fc6072f174c854f0cf)

src/libslic3r/GCode/WipeTower.hpp

@@ -303,6 +303,8 @@ public:
         std::vector<float>  ramming_speed;
         float               nozzle_diameter;
         float               filament_area;
+        float               retract_length;
+        float               retract_speed;
     };