AudioVideo: use avTranscoder

Need to merge "dev_addAvTranscoder" branch to master in sconsProject, to manage the dependency to avTranscoder.
Yes, in OpenFX the image size is dynamic (width/height can change at each frame).
So if you need that information, you're right.

Need to add a check if width/height change from that initial value...
And need to add an option to force the image size...