Vda ffmpeg

This adds support for VDA decoding using ffmpeg's builting decoder.

The ffmpeg one have the benefit of not needing to know width/height beforehand. It's synchronous, so maybe it's slower I don't really know.

To test the ffmpeg one you need to modify CDVDCodecFactory.cpp and disable usage of the standard one (or use a livetv source that doesn't provide width/height).