Iron the purge start out through the skip-point gap in WipeTower2

Port the BBL tower's entry line ironing: extrude the first 3 mm of the
purge, retract, drag the nozzle 1.5x back out through the wall gap at
F600, creep back at F240 and unretract, so the toolchange start blob
ends up in the gap instead of on the wall. Fires only when the purge
starts at the left-edge entry heading right (in-place toolchangers);
SEMM ram/cooling wipes start mid-box and the priming line has no wall,
so both keep their previous output.

src/libslic3r/GCode/WipeTower2.hpp

@@ -361,7 +361,8 @@ private:
 		WipeTowerWriter2 &writer,
 		const WipeTower::box_coordinates  &cleaning_box,
 		float wipe_volume,
-        bool interface_layer);
+        bool interface_layer,
+        bool priming = false);
 
 
     Polygon generate_support_rib_wall(WipeTowerWriter2&                 writer,