Support streaming with zms (using ffmpeg) better
    * ZoneMinder compiles with ffmpeg 0.5..master
    * Make VideoStream class able to stream through ffmpeg again.
    * Aility to use fixed quality instead of fixed bitrate (by specifying bitrate 0...100)
    * Format url parameter supports <format>/<codec> syntax

Details:
    - Redefine av_err2str to a to avoid compiler warning on newer g++'s
    - When using rtp format; if ffmpeg does not have a default codec, use CODEC_ID_MPEG4. (because this is what the default is as of 2.2 when this code was written)
    - Specify ofc->packet_size if rtp format is requested
    - Video generated in a thread to guarantee a constant fps.
    - Move _AVCODECID definition into zm_ffmpeg.h (and use instead of (AV)CodecID
    - Call avformat_network_init because ffmpeg warns about it beeing required soon.
    - increase VideoStream::video_outbuf_size to 4MiB to be able to encode 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.