wasapi: Rewrite most of the code to make it work

wasapi: Rewrite most of the code to make it work

Both the source and the sink elements were broken in a number of ways:

* prepare() was assuming that the format was always S16LE 2ch 44.1KHz.
  We now probe the preferred format with GetMixFormat().
* Device initialization was done with the wrong buffer size
  (buffer_time is in microseconds, not nanoseconds).
* sink_write() and src_read() were just plain wrong and would never
  write or read anything useful.
* Some functions in prepare() were always returning FALSE which meant
  trying to use the elements would *always* fail.
* get_caps() and delay() were not implemented at all.

TODO: support for >2 channels
TODO: pro-audio low-latency
TODO: SPDIF and other encoded passthroughs

Three new properties are now implemented: role, mute, and device.

* 'role' designates the stream role of the initialized device, see:
   https://msdn.microsoft.com/en-us/library/windows/desktop/dd370842(v=vs.85).aspx
* 'device' is a system-wide GUIDesque string for a specific device.
* 'mute' is a sink property and simply mutes it.

On my Windows 8.1 system, the lowest latency that works is:

  wasapisrc buffer-time=20000
  wasapisink buffer-time=10000

aka, 20ms and 10ms respectively. These values are close to the lowest
possible with the IAudioClient interface. Further improvements require
porting to IAudioClient2 or IAudioClient3.

https://docs.microsoft.com/en-us/windows-hardware/drivers/audio/low-latency-audio