Move support cubic infill to separate class.

Support infill is enabled in the GUI.

src/libslic3r/Fill/FillBase.cpp

@@ -39,6 +39,7 @@ Fill* Fill::new_from_type(const InfillPattern type)
     case ipHilbertCurve:        return new FillHilbertCurve();
     case ipOctagramSpiral:      return new FillOctagramSpiral();
     case ipAdaptiveCubic:       return new FillAdaptive();
+    case ipSupportCubic:        return new FillSupportCubic();
     default: throw std::invalid_argument("unknown type");
     }
 }