This commit implements support for PCE (Program Configuration Elements) in the
AAC encoder, and as such allows for encoding of channel layouts not present
in the presets defined by the spec (which only lists the 8 most common ones).

This has been a highly requested feature and is also the first open source encoder
to support this many layouts.

Many thanks to pkviet <pkv.stream@gmail.com> who implemented support for and
verified all channel layouts.