Further rework of ClipperUtils: Replaced many to_polygons() /

to_expolygons() calls with templated ClipperUtils variants to avoid
memory allocation and copying.

src/libslic3r/SLA/SupportPointGenerator.hpp

@@ -90,7 +90,7 @@ public:
         float overlap_area(const Structure &rhs) const { 
             double out = 0.;
             if (this->bbox.overlap(rhs.bbox)) {
-                Polygons polys = intersection(to_polygons(*this->polygon), to_polygons(*rhs.polygon), false);
+                Polygons polys = intersection(*this->polygon, *rhs.polygon, false);
                 for (const Polygon &poly : polys)
                     out += poly.area();
             }