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

The method is now public.

…tion

…reference

Because the _transform attribute could be NULL.

Added getCurrentOutputDuration private method.

* 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.

The same log exists in switchToGeneratorDecoder method.

…ator but cannot

No generator will be used for this stream, but the process is not
stopped.

* Need to check if duration of all output streams are almost equals to
the expected value. But we cannot ensure to have the exact value for all
streams (it may not be possible depending on the fps/rate of input
streams).
* And the result vary depending on the ffmpeg version.

…h frame