Draft: Add the support for VA jpeg/mjpeg decoder.

The jpeg/mjpeg decoder may still be needed.

The digital camera and some mobile phone devices may not have the inter picture compression ability and just compress the video data as mjpeg format. So there are requirements of gst pipeline to do the transcoding for mjpeg streams.

We need to add a vajpegdec in the VA plugins. Does it need a jpegdecoder base class as the other codecs, or just inherit the vidoedecoder because it is simple and there are no relationship between frames(so already stateless)?