Public issue https://github.com/mackron/miniaudio/issues/355

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

This allows the engine to be used independent of a device which might
be useful for tools.

  * ma_gainer
  * ma_panner
  * ma_fader
  * ma_spatializer

These are amalgamated into miniaudio.h now and are no longer required
in the extras folder.