wasapi:
- fixed bug when Exclusive mode was wrongly opening Stereo stream if Mono stream was requested (note: WASAPI device usually does not support Mono in Exclusive mode), resulted in distorted sound
- implemented workaround for WASAPI limitation - in Exclusive mode Mono stream is not supported: now PA creates internal audio device in Stereo mode, and after callback call Mono data is mixed into Stereo output, this workaround is not supported for Blocking mode. all this is done transparent for user, automatically.