Fix of #8437 - Very tiny unprintable polygons caused InfillFailedException in the lightning infill.

src/libslic3r/Fill/Lightning/Generator.cpp

@@ -68,6 +68,8 @@ void Generator::generateInitialInternalOverhangs(const PrintObject &print_object
 
         // Remove the part of the infill area that is already supported by the walls.
         Polygons overhang = diff(offset(infill_area_here, -float(m_wall_supporting_radius)), infill_area_above);
+        // Filter out unprintable polygons and near degenerated polygons (three almost collinear points and so).
+        overhang = opening(std::move(overhang), SCALED_EPSILON, SCALED_EPSILON);
 
         m_overhang_per_layer[layer_nr] = overhang;
         infill_area_above = std::move(infill_area_here);