While this assert() was useful when we had to track the object's state
manually, there was no practical purpose other than verifying old
code, and it complicates our new C++ code.

This new library is integrated in the PcmExport class and (if enabled)
converts MPD's channel order (= FLAC channel order) to ALSA channel
order.

This fixes:
 http://bugs.musicpd.org/view.php?id=3147
and
 http://bugs.musicpd.org/view.php?id=3255

Move code from the "vorbis" decoder.

Return false on error, not nullptr.

Simplify the definition of many build options.

The old formula calculates the output buffer size with "regular"
rounding (to the nearest integer), however sometimes, that is
insufficient and the last sample cannot be resampled.  This causes
audible distortions.  By changing the formula to consider the worst
case (always round up), this problem is eliminated.