shorten: support signal to noise ratio mode (lossy coding)

[ami_stuff]

currently the output is incorrect (audio distortion or silence)

shorten -n 8 test.wav n8.shn

​http://www.datafilehost.com/download-b8386697.html

  -n 256   minimum signal to noise ratio in dB (256 == lossless coding)

C:\ffmpeg -i n8.shn out.wav
ffmpeg version N-40511-g66337bf Copyright (c) 2000-2012 the FFmpeg developers
  built on May  9 2012 02:38:42 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. 50.100 / 51. 50.100
  libavcodec     54. 21.100 / 54. 21.100
  libavformat    54.  4.100 / 54.  4.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 72.105 /  2. 72.105
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 11.100 /  0. 11.100
  libpostproc    52.  0.100 / 52.  0.100
[shorten @ 0x1dc2320] 2 header bytes unparsed
[shn @ 0x1dcc5c0] Estimating duration from bitrate, this may be inaccurate
Input #0, shn, from 'n8.shn':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Audio: shorten, 44100 Hz, 2 channels, s16
Guessed Channel Layout for  Input Stream #0.0 : stereo
Output #0, wav, to 'out.wav':
  Metadata:
    encoder         : Lavf54.4.100
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 44100 Hz, stereo, s16
, 1411 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (shorten -> pcm_s16le)
Press [q] to stop, [?] for help
[shorten @ 0x1dc2320] 2 header bytes unparsed
Input stream #0:0 frame changed from rate:44100 fmt:s16 ch:2 chl:0x0 to rate:441
00 fmt:s16 ch:2 chl:0x3
Multiple frames in a packet from stream 0
size=    1724kB time=00:00:10.00 bitrate=1411.5kbits/s
video:0kB audio:1724kB global headers:0kB muxing overhead 0.002606%

[Carl Eugen Hoyos]

Fixed by Paul B Mahol.