Field-encoded MPEG-2 samples allow to seek to a non-key frame / show artefacts

[Carl Eugen Hoyos]

Some field-encoded mpeg2video samples do not skip non-keyframes when seeking.

$ ffmpeg -ss 1 -i Ondine_clip_cut.mpg -vframes 1 out.jpg
ffmpeg version N-63598-gca2ec98 Copyright (c) 2000-2014 the FFmpeg developers
  built on May 30 2014 13:42:12 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      52. 87.100 / 52. 87.100
  libavcodec     55. 65.100 / 55. 65.100
  libavformat    55. 42.100 / 55. 42.100
  libavdevice    55. 13.101 / 55. 13.101
  libavfilter     4.  5.100 /  4.  5.100
  libswscale      2.  6.100 /  2.  6.100
  libswresample   0. 19.100 /  0. 19.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, mpeg, from 'Ondine_clip_cut.mpg':
  Duration: 00:00:03.18, start: 0.278144, bitrate: 6440 kb/s
    Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p(tv, bt470bg), 720x576 [SAR 16:15 DAR 4:3], max. 9000 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc
    Stream #0:1[0x80]: Audio: ac3, 48000 Hz, stereo, fltp, 192 kb/s
[swscaler @ 0x27e0380] deprecated pixel format used, make sure you did set range correctly
Output #0, image2, to 'out.jpg':
  Metadata:
    encoder         : Lavf55.42.100
    Stream #0:0: Video: mjpeg, yuvj420p, 720x576 [SAR 16:15 DAR 4:3], q=2-31, 200 kb/s, 25 fps, 90k tbn, 25 tbc
    Metadata:
      encoder         : Lavc55.65.100 mjpeg
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg2video -> mjpeg)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=1.6 Lsize=N/A time=00:00:00.04 bitrate=N/A
video:7kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

out.jpg is gray.

[Carl Eugen Hoyos]

Regression since 299fd396
Related to ​issue 1226

[ami_stuff]

here are some potentially useful samples which I created in the past:

​http://www.megafileupload.com/8jss/interlaced_field.7z