IAttributes::getAttributeAs functions now can have a customizable default-parameter to return when attributeName is not found. This makes it easier to work correctly with old serialized data when adding new variables. Also it would probably be worth it passing generally in all serialization the old value as default-parameter, then it can handle missing serialization data as well.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4427 dfc29bdd-3216-0410-991c-e03cc46cb475