Revert "Use number_of_parts for is_splittable"

It is too dangerous to rely on the admesh flag without inspecting the
admesh code line by line and a through test.

This reverts commit cd3cec3e45.

src/libslic3r/Model.cpp

@@ -1467,7 +1467,11 @@ int ModelVolume::extruder_id() const
 
 bool ModelVolume::is_splittable() const
 {
-    return mesh.stl.stats.number_of_parts > 1;
+    // the call mesh.is_splittable() is expensive, so cache the value to calculate it only once
+    if (m_is_splittable == -1)
+        m_is_splittable = (int)mesh.is_splittable();
+
+    return m_is_splittable == 1;
 }
 
 void ModelVolume::center_geometry()

src/libslic3r/Model.hpp

@@ -420,6 +420,12 @@ private:
     TriangleMesh             m_convex_hull;
     Geometry::Transformation m_transformation;
 
+    // flag to optimize the checking if the volume is splittable
+    //     -1   ->   is unknown value (before first cheking)
+    //      0   ->   is not splittable
+    //      1   ->   is splittable
+    mutable int               m_is_splittable{ -1 };
+
 	ModelVolume(ModelObject *object, const TriangleMesh &mesh) : mesh(mesh), m_type(ModelVolumeType::MODEL_PART), object(object)
     {
         if (mesh.stl.stats.number_of_facets > 1)

src/libslic3r/TriangleMesh.cpp

@@ -338,6 +338,19 @@ void TriangleMesh::rotate(double angle, Point* center)
     this->translate(c(0), c(1), 0);
 }
 
+/**
+ * Calculates whether or not the mesh is splittable.
+ */
+bool TriangleMesh::is_splittable() const
+{
+    std::vector<bool> visited;
+    find_unvisited_neighbors(visited);
+
+    // Try finding an unvisited facet. If there are none, the mesh is not splittable.
+    auto it = std::find(visited.begin(), visited.end(), false);
+    return it != visited.end();
+}
+
 /**
  * Visit all unvisited neighboring facets that are reachable from the first unvisited facet,
  * and return them.

src/libslic3r/TriangleMesh.hpp

@@ -68,6 +68,7 @@ public:
     size_t facets_count() const { return this->stl.stats.number_of_facets; }
     bool   empty() const { return this->facets_count() == 0; }
 
+    bool is_splittable() const;
     std::deque<uint32_t> find_unvisited_neighbors(std::vector<bool> &facet_visited) const;
 
     stl_file stl;