Decoding raw packets without streams using codecs that needs extradata

For decoding raw packets without streams, some codecs need a few extradata (like Huffman tables). In this commit, I provided two functions, one for extracting these data and the other for setting them when we want to decode without a stream.

An example usage:

import av

container = av.open('somevideo.mp4')

# Find index of the video stream 
video_stream_index = 0
for index, stream in enumerate(container.streams):
    if isinstance(stream, av.video.stream.VideoStream):
        video_stream_index = index
        break

buffered_packets = []
count = 0
now = False
for packet in container.demux(streams=video_stream_index):
    count += 1
    # We only buffer some packets (here from 100 to 500)
    if count > 100:
        new_packet = av.packet.Packet(input=packet.to_bytes())
        # First packet need to be a keyframe
        if packet.is_keyframe or now:
            buffered_packets.append(new_packet)
            now = True

    if count > 500:
        break

codec_name = container.streams[video_stream_index].codec_context.name
codec_origin = container.streams[video_stream_index].codec_context
codec_new = av.codec.CodecContext.create(codec_name, 'r')


extradata = codec_origin.extradata
if not extradata is None:
    codec_new.extradata = extradata

for packet in buffered_packets:
    try:
        for frame in codec_new.decode(packet):
            frame.to_image().save('frame-%04d.jpg' % frame.index)
    except av.AVError as e:
        print(e)