ao_coreaudio: move AudioStreamChangeFormat to common file and refactor
ao_coreaudio: refactor play
ao_coreaudio: ca_msg: add trailing \n where missing
ao_coreaudio: remove a fixme since that seems fixed
ao_coreaudio: extract mixmode set/unset in utility functions
ao_coreaudio: refactor uninit
ao_coreaudio: refactor play/pause