ffmpeg/4.x: update FFmpeg plugin
ffmpeg/4.x: update FFmpeg plugin

ffmpeg: 4.1.3 -> 4.2.1
libvpx: 1.8.0 -> 1.8.1

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv