aaccoder: add intensity stereo coding support for the trellis quantizer

This commit extends the trellis quantizer (used by the default twoloop coder)
 to accept and correctly encode codebooks needed for intensity stereo and perceptual noise substitution.

Reviewed-by: Claudio Freire <klaussfreire@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>