ENH: use spiral lift only when it is needed

If a travel path goes through an overhang expolygons, and their distance
is shorter than threshold, lift type will be set to SpiralLift.

Signed-off-by: yifan.wu <yifan.wu@bambulab.com>
Change-Id: I345788711755dd8611ecf385818e6052cd8abe9e

src/libslic3r/Layer.hpp

@@ -143,6 +143,9 @@ public:
     ExPolygons 				 lslices;
     std::vector<BoundingBox> lslices_bboxes;
 
+    // BBS
+    ExPolygons              loverhangs;
+
     size_t                  region_count() const { return m_regions.size(); }
     const LayerRegion*      get_region(int idx) const { return m_regions[idx]; }
     LayerRegion*            get_region(int idx) { return m_regions[idx]; }