AVWriterPlugin: refactoring functions to update OFX audio parameters

Less code to maintain.
AVWriterPlugin: fix input image of transcode

The input image of transcode is the result of gil conversion. We set a
pixel to describe it: RGB, etc...
AVWriterPlugin: fix when choose a specific pixel format

* Get the pixel format in the list of supported pixel formats.
* No need to store the AVPixelFormat.
AVWriterPlugin: fix when choose a specific sample format

* Get the sample format in the list of supported sample formats.
* No need to store the AVSampleFormat.
* Factory: no default value for the corresponding OFX Choice.