Undo / Redo memory conservation strategy: Release recoverable data

starting from the objects of lowest ObjectID.
(convex hulls are recoverable as well as the indexed triangle sets
inside the TriangleMeshes or the triangle connectivity information).

Now the top most snapshot (the temp one taken before Undo jump) will
never be released.

src/libslic3r/TriangleMesh.cpp

@@ -613,6 +613,34 @@ size_t TriangleMesh::memsize() const
 	return memsize;
 }
 
+// Release optional data from the mesh if the object is on the Undo / Redo stack only. Returns the amount of memory released.
+size_t TriangleMesh::release_optional()
+{
+	size_t memsize_released = sizeof(stl_neighbors) * this->stl.neighbors_start.size() + this->its.memsize();
+	// The indexed triangle set may be recalculated using the stl_generate_shared_vertices() function.
+	this->its.clear();
+	// The neighbors structure may be recalculated using the stl_check_facets_exact() function.
+	this->stl.neighbors_start.clear();
+	return memsize_released;
+}
+
+// Restore optional data possibly released by release_optional().
+void TriangleMesh::restore_optional()
+{
+	if (! this->stl.facet_start.empty()) {
+		// Save the old stats before calling stl_check_faces_exact, as it may modify the statistics.
+		stl_stats stats = this->stl.stats;
+		if (this->stl.neighbors_start.empty()) {
+			stl_reallocate(&this->stl);
+			stl_check_facets_exact(&this->stl);
+		}
+		if (this->its.vertices.empty())
+			stl_generate_shared_vertices(&this->stl, this->its);
+		// Restore the old statistics.
+		this->stl.stats = stats;
+	}
+}
+
 void TriangleMeshSlicer::init(const TriangleMesh *_mesh, throw_on_cancel_callback_type throw_on_cancel)
 {
     mesh = _mesh;