Return an error for MOut on a stereo stream.

It seems like it won't downmix, it returns an error.
Handle atrac files with larger "fact" chunks.

Ends up with the a separate offset for loops, it seems like.  This corrects
looping information for these files.
Correct atrac looping offset by one frame.
Merge pull request #6959 from unknownbrackets/atrac-minor

Correct some minor issues with atrac looping