Fix of [2.3.0-alpha4] Crash - several models cause crash when slicing #5208

Fixed some issues in internal anchors of the Adaptive Cubic infill.
The ugly and dangerous implicit casting operators in Line, MultiPoint,
Polyline and Polygon were made explicit.

src/libslic3r/SupportMaterial.cpp

@@ -2513,7 +2513,7 @@ void LoopInterfaceProcessor::generate(MyLayerExtruded &top_contact_layer, const
                 Polygon     &contour = (i_contour == 0) ? it_contact_expoly->contour : it_contact_expoly->holes[i_contour - 1];
                 const Point *seg_current_pt = nullptr;
                 coordf_t     seg_current_t  = 0.;
-                if (! intersection_pl(contour.split_at_first_point(), overhang_with_margin).empty()) {
+                if (! intersection_pl((Polylines)contour.split_at_first_point(), overhang_with_margin).empty()) {
                     // The contour is below the overhang at least to some extent.
                     //FIXME ideally one would place the circles below the overhang only.
                     // Walk around the contour and place circles so their centers are not closer than circle_distance from each other.