Wipe tower: reorder extruders so first layer starts with soluble if possible

That way it will not be wiped on first layer

src/libslic3r/GCode/ToolOrdering.cpp

@@ -329,6 +329,16 @@ void ToolOrdering::reorder_extruders(unsigned int last_extruder_id)
                     lt.extruders.front() = last_extruder_id;
                     break;
                 }
+
+            // On first layer with wipe tower, prefer a soluble extruder
+            // at the beginning, so it is not wiped on the first layer.
+            if (lt == m_layer_tools[0] && m_print_config_ptr && m_print_config_ptr->wipe_tower) {
+                for (size_t i = 0; i<lt.extruders.size(); ++i)
+                    if (m_print_config_ptr->filament_soluble.get_at(lt.extruders[i]-1)) { // 1-based...
+                        std::swap(lt.extruders[i], lt.extruders.front());
+                        break;
+                    }
+            }
         }
         last_extruder_id = lt.extruders.back();
     }