wasapi: first stab of audio output using the Windows Audio Session API

This is _very_ incomplete and buggy:
 - no time synchronization,
 - no sample format, sample rate and channels mask negotiation,
 - no channels reordering,
 - no S/PDIF and HDMI pass-through,
 - no volume control,
 - no device selection,
 - no fail-over to another device when needed,
 - leaking COM thread data at exit (missing dedicated thread),
 - incomplete build system integration.

But it compiles (with mingw-w64) and outputs working audio.