aaccoder: add intensity stereo support to encode_window_bands_info quantizer

This commit adds support for both PNS and IS (intensity stereo) codebooks to the
encode_window_bands_info() quantizer, used by the faast, faac and anmr non-default,
native coders. This does not mean that both extensions now work with those coders,
some are simply unsuited and will trigger an assertion in the encoder while
others simply ignore the changed scalefactor indices and band types.
This commit simply adds support for encoding said band types with the alternative
coders. Future commits to the coders will be required to make them suitable.

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