The process case of a StreamTranscoder could change during the process
(depending on the state of the _currentDecoder attribute).

* To stop the process when all the input data are read.
* This is the default process method.
* Do not check the output program duration when we use this process
policy.

The method is now public.

* Try to process a new frame for each stream (do not exit the method if
a stream failed to process).
* Skip the generated streams because they always succeed.