Make nameFor functions in IColladaMeshWriterNames non-const as it's not necessary in the writer and we don't know what people will do with it (and because I already needed it non-const to avoid ugly mutable variables in writing the example).

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4269 dfc29bdd-3216-0410-991c-e03cc46cb475