Replace custom modified discrete cosine transform with ffmpeg's own.

Replace custom modified discrete cosine transform with ffmpeg's own.
This does alter the decoded output, but not by much.
The new output is closer to that produced by Real's "official" decoder,
and the decoder runs slightly faster.

Patch by Ian Braithwaite ian at braithwaite dot dk

Originally committed as revision 8325 to svn://svn.ffmpeg.org/ffmpeg/trunk