Fix for a problem with inverted sign of output data from ffvorbis decoder.

Now the sign of audio samples in ffvorbis output is the same as in original
uncompressed audio file and this also allows the use of tiny_psnr to compare
ffvorbis with libvorbis/tremor.

Originally committed as revision 18878 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vorbis_dec.c

@@ -896,8 +896,8 @@ static int vorbis_parse_id_hdr(vorbis_context *vc){
     vc->saved           = av_mallocz((vc->blocksize[1]/4)*vc->audio_channels * sizeof(float));
     vc->previous_window=0;
 
-    ff_mdct_init(&vc->mdct[0], bl0, 1, 1.0);
-    ff_mdct_init(&vc->mdct[1], bl1, 1, 1.0);
+    ff_mdct_init(&vc->mdct[0], bl0, 1, -1.0);
+    ff_mdct_init(&vc->mdct[1], bl1, 1, -1.0);
 
     AV_DEBUG(" vorbis version %d \n audio_channels %d \n audio_samplerate %d \n bitrate_max %d \n bitrate_nom %d \n bitrate_min %d \n blk_0 %d blk_1 %d \n ",
             vc->version, vc->audio_channels, vc->audio_samplerate, vc->bitrate_maximum, vc->bitrate_nominal, vc->bitrate_minimum, vc->blocksize[0], vc->blocksize[1]);