ac3enc: shift coefficients to 24-bit following MDCT rather than using an exponent...

ac3enc: shift coefficients to 24-bit following MDCT rather than using an exponent offset.

This makes channel coupling more accurate, increasing quality for stereo
content.  It also simplifies exponent extraction and mantissa quantization
by no longer needing to apply an offset to the exponents.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>