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/admesh/connect.cpp

@@ -430,6 +430,8 @@ private:
 // floats of the first edge matches all six floats of the second edge.
 void stl_check_facets_exact(stl_file *stl)
 {
+	assert(stl->facet_start.size() == stl->neighbors_start.size());
+
   	stl->stats.connected_edges         = 0;
   	stl->stats.connected_facets_1_edge = 0;
   	stl->stats.connected_facets_2_edge = 0;