Support seeking to arbitrary positions

[mbauman]

It'd be nice to be able to add random access to the API here. The following works for my configuration, but I'm not submitting it as a PR because:

• it has an off-by-factor-of-two error in the conversion from time (in seconds) to the integer time that I couldn't track down. (found it)
• I'm uncertain how to verify that it will work in all your supported configurations.
• It's blind monkey-see-monkey-do code based upon seekstart; I don't know what these things are actually doing.

Feel free to use this (or not) however you see fit.

function Base.seek(s::VideoIO.VideoReader, time, video_stream=1)
    pCodecContext = s.pVideoCodecContext
    seek(s.avin, time, video_stream)
    VideoIO.avcodec_flush_buffers(pCodecContext)
    s
end
function Base.seek(avin::VideoIO.AVInput, time, video_stream = 1)
    # AVFormatContext
    fc = avin.apFormatContext[1]

    # Get stream information
    stream_info = avin.video_info[video_stream]
    seek_stream_index = stream_info.stream_index0
    stream = stream_info.stream
    time_base = stream_info.codec_ctx.time_base
    ticks_per_frame = stream_info.codec_ctx.ticks_per_frame

    # Seek
    ret = VideoIO.av_seek_frame(fc, seek_stream_index, convert(Int, div(time*time_base.den, time_base.num*ticks_per_frame)), VideoIO.AVSEEK_FLAG_ANY)

    ret < 0 && throw(ErrorException("Could not seek to start of stream"))

    return avin
end

[mbauman]

Whoops, this is a duplicate of #30.