Move polygons instead of copying them on move

MultiPoint had no rvalue constructor, so the derived move constructors
bound to the const reference and copied; append reserved exactly, so
collecting pieces one by one was quadratic. Colour segmentation ~3 s at
0.1 mm / 2000k, was ~40, and ordinary prints gain too.

src/libslic3r/Support/SupportMaterial.cpp

@@ -929,9 +929,9 @@ public:
                 ::fread(&y, sizeof(coord_t), 1, file);
                 poly.points.emplace_back(Point(x * scale, y * scale));
             }
+            printf("Polygon %d, area: %lf\n", i, area(poly.points));
             if (which == -1 || which == i)
                 m_support_polygons_deserialized.emplace_back(std::move(poly));
-            printf("Polygon %d, area: %lf\n", i, area(poly.points));
         }
         ::fread(&n_polygons, 4, 1, file);
         m_trimming_polygons_deserialized.reserve(n_polygons);