AVWriterFactory: update interface

* Rename some label of OFX params to simplify the interface of the
plugin.
AVWriter: prefix options name

* Use prefix to have a different name for each new OFX param (even if
AVOptions have the same name).
* Details:
     * format category: 'f_'
     * video category: 'v_'
     * audio category: 'a_'
     * metadatas category: 'm_'
* Refactoring: update label of audioCustomGroupParam.
AVReader: prefix options name

* Use prefix to have a different name for each new OFX param (even if
AVOptions have the same name).
* Details:
     * format category: 'f_'
     * video category: 'v_'
     * audio category: 'a_'
     * metadatas category: 'm_'
AVWriter: add audioCopyStreamParam

* Add this OFX param to each audioSubGroup.
* If the boolean is set:
     * hide the audio preset param of the audioSubGroup.
     * only rewrap the audio stream.
* If not:
     * transcode the audio stream.
AVWriter: fix cleanVideoAndAudio

* Fix seg fault when create a graph saved by Nuke, and immediatly
process.
AVReader: fit to new avTranscoder API
AVReader: fix seek

* Do not seek when reading the first frame.
* Update message printed at each frame (tmp for debug).
AVReaderProcess: render a black image if can't get pixels of the input

This avoid seg faults if there is a problem when reading the input
image.
AVWriter: fit to new avTranscoder API

* Use a Transcoder for audio instead of:
     * an array of InputFile.
     * an array of InputStreamAudio.
     * an array of DummyInputStream.
     * an array of OutputStreamAudio.
     * an AudioTransform.
     * an array of stream id.
* Warning: problem with the colorTransform. Waiting for avTranscoder to
encode video AND audio with an instance of a Transcoder.