In the case where the user want to roll-back a convertor we should first rewind the
convertor to the beginning and then move it to the requested position. Otherwise the
logic do roll-back a convertor in an optimized way is really difficult. BTW, in terms of
performances one would prefer to have 2 convertors and play with position inside, than
just having one and roll it back.
This commit was SVN r7378.
ec621c2145