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/Fill/FillBase.hpp

@@ -34,14 +34,15 @@ struct FillParams
 {
     bool        full_infill() const { return density > 0.9999f; }
     // Don't connect the fill lines around the inner perimeter.
-    bool        dont_connect() const { return anchor_length < 0.05f; }
+    bool        dont_connect() const { return anchor_length_max < 0.05f; }
 
     // Fill density, fraction in <0, 1>
     float       density 		{ 0.f };
 
     // Length of an infill anchor along the perimeter.
     // 1000mm is roughly the maximum length line that fits into a 32bit coord_t.
-    float       anchor_length   { 1000.f };
+    float       anchor_length       { 1000.f };
+    float       anchor_length_max   { 1000.f };
 
     // Don't adjust spacing to fill the space evenly.
     bool        dont_adjust 	{ true };