This allows reordering, duplication, and silencing of input channels.

Simplifies header dependencies by not including all other internal headers
in internal.h.

This allows AudioMix to be treated the same way as other conversion contexts
and removes the requirement to allocate it at the same time as the
AVAudioResampleContext.

The current matrix get/set functions are split between the public interface
and AudioMix private functions.

Provides an option to disable mix level normalization

Signed-off-by: Diego Biurrun <diego@biurrun.de>

Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

This is a new library for audio sample format, channel layout, and sample rate
conversion.