nuv vp3: seek only to keyframes

[ami_stuff]

​http://samples.mplayerhq.hu/nsv/64vp3.nsv

there are visible artefacts in the output file

C:\>ffmpeg -ss 3 -i 64vp3.nsv -an out.avi
ffmpeg version N-55393-g3b2e99f Copyright (c) 2000-2013 the FFmpeg developers
  built on Aug  8 2013 21:32:57 with gcc 4.7.3 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetyp
e --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --ena
ble-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-l
ibopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libsp
eex --enable-libtheora --enable-libtwolame --enable-libvo-aacenc --enable-libvo-
amrwbenc --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --
enable-libxvid --enable-zlib
  libavutil      52. 41.100 / 52. 41.100
  libavcodec     55. 24.100 / 55. 24.100
  libavformat    55. 13.102 / 55. 13.102
  libavdevice    55.  3.100 / 55.  3.100
  libavfilter     3. 82.100 /  3. 82.100
  libswscale      2.  4.100 /  2.  4.100
  libswresample   0. 17.103 /  0. 17.103
  libpostproc    52.  3.100 / 52.  3.100
C:\64vp3.nsv: could not seek to position 3.000
Input #0, nsv, from '64vp3.nsv':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: vp3 (VP31 / 0x31335056), yuv420p, 240x192, 5 tbr, 5 tbn,
 5 tbc
    Stream #0:1: Audio: mp3 (MP3  / 0x2033504D), 22050 Hz, mono, s16p, 16 kb/s
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf55.13.102
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 240x192, q=2-31, 200
 kb/s, 5 tbn, 5 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (vp3 -> mpeg4)
Press [q] to stop, [?] for help
[vp3 @ 02bb68c0] vp3: first frame not a keyframe
C:\64vp3.nsv: Operation not permitted
frame=   90 fps=0.0 q=2.0 Lsize=     477kB time=00:00:57.00 bitrate=  68.6kbits/
s
video:465kB audio:0kB subtitle:0 global headers:0kB muxing overhead 2.639593%

[ami_stuff]

still reproducible