NEW: add boolean in export stl

Change-Id: Ibeab33f27ad7a2531bb256edd0e3f853cf8def7c
(cherry picked from commit 6cfb4371a1b9053b54569d91a9c29af97a753c0e)

src/libslic3r/MeshSplitImpl.hpp

@@ -108,6 +108,21 @@ template<class IndexT> struct ItsNeighborsWrapper
     const auto& get_index() const noexcept { return index_ref; }
 };
 
+// Can be used as the second argument to its_split to apply a functor on each
+// part, instead of collecting them into a container.
+template<class Fn>
+struct SplitOutputFn {
+
+    Fn fn;
+
+    SplitOutputFn(Fn f): fn{std::move(f)} {}
+
+    SplitOutputFn &operator *() { return *this; }
+    void           operator=(indexed_triangle_set &&its) { fn(std::move(its)); }
+    void           operator=(indexed_triangle_set &its) { fn(its); }
+    SplitOutputFn& operator++() { return *this; };
+};
+
 // Splits a mesh into multiple meshes when possible.
 template<class Its, class OutputIt>
 void its_split(const Its &m, OutputIt out_it)
@@ -155,7 +170,8 @@ void its_split(const Its &m, OutputIt out_it)
             mesh.indices.emplace_back(new_face);
         }
 
-        out_it = std::move(mesh);
+        *out_it = std::move(mesh);
+        ++out_it;
     }
 }