Unify GCodeGenerator travel to first layer position.

* Fixes SPE-2116. The reason for this bug was forgotten change in
  WipeTowerIntegration after change in gcode.cpp.
* Prevents future similar bugs by unifing the functionality to a method.

src/libslic3r/GCode.cpp

@@ -2947,9 +2947,38 @@ void GCodeGenerator::GCodeOutputStream::write_format(const char* format, ...)
     va_end(args);
 }
 
+std::string GCodeGenerator::travel_to_first_position(const Vec3crd& point) {
+    std::string gcode;
+
+    const Vec3d gcode_point = to_3d(this->point_to_gcode(point.head<2>()), unscaled(point.z()));
+
+    if (!this->last_position) {
+        double lift{
+            EXTRUDER_CONFIG(travel_ramping_lift) ? EXTRUDER_CONFIG(travel_max_lift) :
+                                                   EXTRUDER_CONFIG(retract_lift)};
+        const double upper_limit = EXTRUDER_CONFIG(retract_lift_below);
+        const double lower_limit = EXTRUDER_CONFIG(retract_lift_above);
+        if ((lower_limit > 0 && gcode_point.z() < lower_limit) ||
+            (upper_limit > 0 && gcode_point.z() > upper_limit)) {
+            lift = 0.0;
+        }
+        gcode += this->writer().get_travel_to_z_gcode(gcode_point.z() + lift, "lift");
+    }
+
+    this->last_position = point.head<2>();
+    this->writer().update_position(gcode_point);
+
+    std::string comment{"move to first layer point"};
+    gcode += this->writer().get_travel_to_xy_gcode(gcode_point.head<2>(), comment);
+    gcode += this->writer().get_travel_to_z_gcode(gcode_point.z(), comment);
+
+    m_current_layer_first_position = gcode_point;
+    return gcode;
+}
+
 std::string GCodeGenerator::_extrude(
-    const ExtrusionAttributes       &path_attr, 
-    const Geometry::ArcWelder::Path &path, 
+    const ExtrusionAttributes       &path_attr,
+    const Geometry::ArcWelder::Path &path,
     const std::string_view           description,
     double                           speed)
 {
@@ -2958,27 +2987,7 @@ std::string GCodeGenerator::_extrude(
 
     if (!m_current_layer_first_position) {
         const Vec3crd point = to_3d(path.front().point, scaled(this->m_last_layer_z + this->m_config.z_offset.value));
-        const Vec3d gcode_point = to_3d(this->point_to_gcode(point.head<2>()), unscaled(point.z()));
-
-        if (!this->last_position) {
-            double lift{
-                EXTRUDER_CONFIG(travel_ramping_lift) ? EXTRUDER_CONFIG(travel_max_lift) :
-                                                       EXTRUDER_CONFIG(retract_lift)};
-            const double upper_limit = EXTRUDER_CONFIG(retract_lift_below);
-            const double lower_limit = EXTRUDER_CONFIG(retract_lift_above);
-            if ((lower_limit > 0 && gcode_point.z() < lower_limit) ||
-                (upper_limit > 0 && gcode_point.z() > upper_limit)) {
-                lift = 0.0;
-            }
-            gcode += this->writer().get_travel_to_z_gcode(gcode_point.z() + lift, "lift");
-        }
-
-        this->last_position = path.front().point;
-        this->writer().update_position(gcode_point);
-
-        gcode += this->writer().get_travel_to_xy_gcode(gcode_point.head<2>(), "move to first layer point");
-        gcode += this->writer().get_travel_to_z_gcode(gcode_point.z(), "move to first layer point");
-        m_current_layer_first_position = gcode_point;
+        gcode += this->travel_to_first_position(point);
     } else {
         // go to first point of extrusion path
         if (!this->last_position) {

src/libslic3r/GCode.hpp

@@ -351,6 +351,9 @@ private:
         ExtrusionRole role,
         const std::string &comment
     );
+
+    std::string travel_to_first_position(const Vec3crd& point);
+
     bool            needs_retraction(const Polyline &travel, ExtrusionRole role = ExtrusionRole::None);
 
     std::string     retract_and_wipe(bool toolchange = false);

src/libslic3r/GCode/WipeTowerIntegration.cpp

@@ -72,14 +72,7 @@ std::string WipeTowerIntegration::append_tcr(GCodeGenerator &gcodegen, const Wip
             }
         } else {
             const Vec3crd point = to_3d(xy_point, scaled(z));
-            const Vec3d gcode_point = gcodegen.point_to_gcode(point);
-            gcodegen.last_position = point.head<2>();
-            gcodegen.writer().update_position(gcode_point);
-            gcode += gcodegen.writer()
-                         .get_travel_to_xy_gcode(gcode_point.head<2>(), comment);
-            gcode += gcodegen.writer()
-                         .get_travel_to_z_gcode(gcode_point.z(), comment);
-            gcodegen.m_current_layer_first_position = gcode_point;
+            gcode += gcodegen.travel_to_first_position(point);
         }
         gcode += gcodegen.unretract();
     } else {