ladspa: support M:N channel mapping in plugins
ladspa: support M:N channel mapping in plugins

This allows plugins such as the LADSPA vocoder to work.

Using the formant.wav and carrier.wav samples from
http://www.sirlab.de/linux/descr_vocoder.html the following
command-line is successful:

sox -M formant.wav carrier.wav \
  -b 16 result.wav rate 44100 \
  ladspa vocoder 16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Using 16-bit (or higher) output reduces the noise-floor hiss
greatly since LADSPA processing is done as 32-bit float.

pcqpcq/sox,CaptainHayashi/sox,uklauer/sox,uklauer/sox,cbagwell/sox,CaptainHayashi/sox,CaptainHayashi/sox,cbagwell/sox,CaptainHayashi/sox,pcqpcq/sox,uklauer/sox,uklauer/sox,cbagwell/sox,pcqpcq/sox,pcqpcq/sox,cbagwell/sox