Add support for the following NIF property: NiDitherProperty
In Ashes of Apocolypse, there are many warnings abou NiDitherProperty
Unhandled property type: NiDitherProperty
http://niftools.sourceforge.net/doc/niflib/classNiflib_1_1NiDitherProperty.html
(RM-2398 from redmine: created on 2015-02-23 by bret curtis, , closed on 2015-03-24 by nobody)