shorten: add duration info

[ami_stuff]

hopefully this will make shorten files seekable in ffplay

C:\>ffmpeg -i test.shn
ffmpeg version N-40040-gc047afb Copyright (c) 2000-2012 the FFmpeg developers
  built on Apr 22 2012 02:36:31 with gcc 4.5.0 20100414 (Fedora MinGW 4.5.0-1.fc
14)
  configuration: --prefix=/var/www/users/research/ffmpeg/snapshots/build --arch=
x86 --target-os=mingw32 --cross-prefix=i686-pc-mingw32- --cc='ccache i686-pc-min
gw32-gcc' --enable-w32threads --enable-memalign-hack --enable-runtime-cpudetect
--enable-cross-compile --enable-static --disable-shared --extra-libs='-lws2_32 -
lwinmm' --extra-cflags='--static -I/var/www/users/research/ffmpeg/snapshots/buil
d/include' --extra-ldflags='-static -L/var/www/users/research/ffmpeg/snapshots/b
uild/lib' --enable-bzlib --enable-zlib --enable-gpl --enable-version3 --enable-n
onfree --enable-libx264 --enable-libspeex --enable-libtheora --enable-libvorbis
--enable-libfaac --enable-libxvid --enable-libopencore-amrnb --enable-libopencor
e-amrwb --enable-libmp3lame --enable-libvpx --disable-decoder=libvpx
  libavutil      51. 47.100 / 51. 47.100
  libavcodec     54. 15.100 / 54. 15.100
  libavformat    54.  3.100 / 54.  3.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 71.102 /  2. 71.102
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 11.100 /  0. 11.100
  libpostproc    52.  0.100 / 52.  0.100
[shorten @ 0x1db2320] 2 header bytes unparsed
Input #0, shn, from 'test.shn':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Audio: shorten, 44100 Hz, 2 channels, s16
At least one output file must be specified

[Elon Musk]

Nope, seeking can only work for files with seek tables, and only if seeking is implemented, would need new demuxer ...

[Elon Musk]

Not sure if libavcodec needs to be changed too...