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.

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

Signed-off-by: Martin Storsjö <martin@martin.st>

Adds some validation for changing parameters after setting the matrix and
fixes a bug in the conversion path setup.

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

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