Moved depth_func_ from BaseNode to GL_State since most nodes don't use this. Updated all nodes to reflect this, especially LineNode which can now be set to be "always on top" as is desired by the lasso selection.

Fixed a crash in Lasso Selection that would occur if the line was not properly closed.

git-svn-id: http://www.openflipper.org/svnrepo/OpenFlipper/branches/Free@10091 383ad7c9-94d9-4d36-a494-682f7c89f535