Vda decoding fixups

This is a series of changes for the VDA decoder. For one it adds support for VDA decoding using ffmpeg's builting decoder. It also simplifies the logic of the original VDA decoding codec.

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).