- Fixed the problem that the end iterator could become valid again if it was incremented.
- Decrementing the start iterator will return an invalid iterator now (Maybe we can implement reverse iterators)
- Removed one check from the iteration, which should result in faster execution of is_valid().



git-svn-id: http://www.openmesh.org/svnrepo/OpenMesh/trunk@1225 fdac6126-5c0c-442c-9429-916003d36597