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.

Use the "_format" parameter instead of the (uninitialized) "format"
attribute.

Report unsupported format while opening the filter, not later when the
first conversion takes place.

Fixes assertion failure in destructor by not assigning
{src,dest}_format when an error occurs.

The standard has been renamed since the early draft that was
implemented in MPD.

Necessary for "final" on gcc 4.6.

Unused.  Bit reversing is done in the decoder.

Obtained from the Mercurial repository at
https://code.google.com/p/dsd2pcm/

For the range-based loop on a ConstBuffer object.

Convert to 31 bit first, then right-shift with rounding to 16 bit.