Allow 32 bit platforms to use 32 bit instead of 64 bit for summing 24
bit samples.

This is really just a mono mapper, but the important part is that this
library cannot fail anymore.

Prepare for adding more resamplers.

The PcmResampler interface is implemented by the two classes
FallbackPcmResampler and LibsampleratePcmResampler.  This prepares for
adding more resampler libraries.

Don't use src_format.  In the middle of Convert(), the current
AudioFormat has already been modified, it's now something in between
src_format and dest_format.  This simplifies keeping track of what
remains to be done.

Wrapper for pcm_resample_global_init(), just in case other PCM
libraries need initialization, too.

Remove compatibility code.

Method PcmConvert::Convert() modifies the src_format variable.  This
used to be a parameter, however commit d2679f59 made it an attribute
instead.  The modification to src_format persisted, and the next call
would return garbage.

Using pcm_resample_lsr_32() for 24 bit samples works, but may cause 24
bit overflows.  This commit makes 24 bit a special case with explicit
clipping.

Replaces Reset() and eliminates the AudioFormat parameters from the
Convert() method.

Lighter API.

Avoid NaN to allow -ffast-math.