Fix mipmap overwriting after manually changing automatically generated mipmaps due to changing the base texture and triggering an auto update of the mipmaps. The auto update cannot be re-enabled afterwards for now, though.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3766 dfc29bdd-3216-0410-991c-e03cc46cb475