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

Avoid multiplication.  This is a speedup of 20%.

This is nearly 4 times faster than the "portable" algorithm.

The previous commit eliminated the need for that.

.. instead of reusing FloatToInteger<S24> and converting from S24 to
S32 in-place.

Refactor the conversion functions to classes and pass an instance to
the new function AllocateConvert().

Kludge to work around DSD playback failure because the
ReplayGainFilterPlugin is always in the filter chain.

Use the existing PcmDither library.

Instead of just adding a rectangular random value before shifting back
to the normal scale, use the existing PcmDither library.

This code is unable to dither.  Until we implement that, let's remove
the code for now.  i386 isn't relevant anymore anyway.

16 bit is not enough for volume calculations.

All callers need this shift, so let's move it to the basic method.

Prepare for adding state.

Rename pcm_convert_domain to pcm_domain.  Move it out so we can use it
without depending on the whole PcmConvert library.

Use the SampleTraits template and let the compiler generate a special
case for S32 instead of reusing S24_P32.

Move from PcmClamp().