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.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18878 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b