AVReader: add beginSequenceRender

* In beginSequenceRender:
     * get source image
     * initialisation of Pixel description.
     * get image to decode
=> Stop allocate new images and set up an output pixel in
AVReaderProcess::setup function (at each frame).
AVReader: refactoring

* Supress unused includes.
AVReader: refactoring

* Update comments: add @todo.
* Explain magici number in AVReaderProcess (variable pixelDepthInBytes).
AVWriterFactory: don't add CodecContextOptions to audioGroupTab

* The options from loadCodecContextOptions( AV_OPT_FLAG_ENCODING_PARAM |
AV_OPT_FLAG_AUDIO_PARAM ), added to the audioGroup, are nearly the same
as loadCodecContextOptions( AV_OPT_FLAG_ENCODING_PARAM |
AV_OPT_FLAG_VIDEO_PARAM ), added to the videoGroup.
* The problem: each OFX param has to have a unique name.
* Temporary solution: only add OFX params to the videoGroup (the plugin
is a video writer at first).
* Future solution: use prefix to each option name (video_... and
_audio_...).
AVReaderPlugin: ensureVideoIsOpen when beginSequenceRender
AVWriter: fix list of supported pixel formats

* By default the list contains all pixel formats supported by all video
codecs.
* The list is not correctly updated in Nuke (same problem in the
previous version of the plugin).
* Get all pixel formats if the list is empty with the video codec
indicated.
AVWriter: audio transcoding from several audio files

* Factory:
     * generalize the audio file path and the audio stream id to a
dynamic list of 'audioSubGroup' (an audioSubGroup contains a file path,
a stream id, and a list of audio preset).
     * The number of 'audioSubGroup' depends on the value of
audioNbStream param.
* Definitions:
     * set the maximum of 'audioSubGroup' (for now equal to the maximum
of audio stream in the output file).
* Plugin:
     * manage the dynamic list of 'audioSubGroup'.
     * AVProcessParams struct contains an array of input audio file path
(instead of one path).
     * _outputStreamVideo is no longer a pointer.
     * add beginSequenceRender to clean output file and video / audio
data before a new render.
     * fix: call beginWrap to the output file only once (use a flag:
_initOutpuFile).
* Process:
     * transcode the list of audio streams added to the outputFile.
AVWriter: add encode of silent traks

* Use DummyInputStream objects from avTranscoder.