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.

All parameters seem mandatory, otherwise this call makes MPD crash.