aacenc: Add support for Perceptual Noise Substitution energy values

This commit implements support for writing the noise energy values used in PNS.
The difference between regular scalefactors and noise energy values is that the latter
require a small preamble (NOISE_PRE + energy_value_diff) to be written as the first
noise-containing band. Any following noise energy values use the previous one to
base their "diff" on. Ordinary scalefactors remain unchanged other than that they ignore the noise values.

This commit should not change anything by itself, the following commits will bring it in use.

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