Implemented Outer/Outer/inner wall sequence for classic slicer as Out…

…er/Inner mode, but first and second external perimeters are swapped. This improves overhang performance at the cost of a slight decrease in print quality

src/libslic3r/PerimeterGenerator.cpp

@@ -1738,6 +1738,24 @@ void PerimeterGenerator::process_classic()
                     this->object_config->brim_type == BrimType::btOuterOnly &&
                     this->object_config->brim_width.value > 0))
                 entities.reverse();
+            else if (this->config->wall_sequence == WallSequence::OuterOuterInner)
+                if (entities.entities.size() > 1) {
+                    entities.reverse();
+
+                    int swapped = 0;
+                    int index;
+
+                    for (index=0; index < entities.entities.size(); index++) {
+                        if (entities.entities[index]->role() == erExternalPerimeter) {
+                            if (swapped == 0 && (index+1) < entities.entities.size()) {
+                                std::swap(entities.entities[index], entities.entities[index+1]);
+                                swapped = 1;
+                            }
+                        } else {
+                            swapped = 0;
+                        }
+                    }
+                }
             // SoftFever: sandwich mode 
             else if (this->config->wall_sequence == WallSequence::InnerOuterInner)
                 if (entities.entities.size() > 1){