Tweaks to the WASAPI backend.

This commit attempts to address a crackling issue in full-duplex mode.

* When the capture buffer becomes over-filled it will discard some
  frames to prevent it from straddling the capacity of the internal
  buffer and causing excessive overruns.
* A fix has been implemented to the function that retrieves the number
  of available frames in the device's internal buffer.
* Low-latency shared mode (IAudioClient3) forces small buffer sizes so
  a change has been implemented which bypasses low-latency shared mode
  when a large buffer is specified.