[FFmpeg-devel] OSX AudioToolbox codec support

[Rodger Combs]

This series adds decoders and encoders based on OSX's AudioToolbox framework.
They may be faster than the native ones (particularly in cases where they can
be hardware-accelerated), the encoders are competitive on quality, and some
users may find them convenient from a licensing perspective.

ADPCM_IMA_QT can also be encoded by AudioToolbox, but I couldn't figure out how
to get it working properly, so it's left with just the decoders.

I'm not sure if the alaw/ulaw and IMA codecs are worth having at all since
they're so simple; I just threw them in since they were easy.

The ALAC encoder and decoder have been tested as bitexact.

The decoder delay handling is pretty awkward and potentially wrong; I wish we
had a version of audio_frame_queue for decoders.