StreamTranscoder: manage end of encoding

* Transcoder: add init function to initialize all streams.
* StreamTranscoder: add init function to pre-process frames necessary to
delete the latency.
* OutputEssence: add a CodedDesc and its getter function. With that, we
can get the description of essence whithout knowing its type (video,
audio...).
* Update OutputVideo and OutputAudio to getVideoDesc and getAudioDesc by
cast the CodedDesc in OutputEssence.
* CodedDesc: add getLatency.
* Update avProcessor app.: init the transcoder before start transcode.
* fix #49