Make the audio_format argument non-const.  Allow the open() method to
modify it, to indicate that it wants a different input audio format
than the one specified.  Check that condition in chain_filter_open(),
and fail.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The filter API allows us to implement software volume as a pluggable
filter, and we will be able to integrate libraries like SoX.