Extend now-private portions of API to store channel matrixing data

Add a number of output-channel ordering defines to public interface; 
these are intended to be ORed with format->channels to request specific 
output channel behavior.  The old behavior of passing through with no 
matrixing is thus preserved as the default for apps currently relying on 
that behavior.



git-svn-id: http://svn.xiph.org/trunk/ao@16795 0101bb08-14d6-0310-b084-bc0e0c8e3800