* move _AVCODECID definition into zm_ffmpeg.h (and use instead of (AV)CodecID
* call avformat_network_init because ffmpeg says so
* increase VideoStream::video_outbuf_size to 4MiB to be able to decode larger pictures
* buffer X packet before starting to stream video to client to provide smoother playback
  experimental for now. currently 1 packet is beeing buffered; which should make no difference.