ao_wasapi: Add device latency to get_delay

The lack of device latency made get_delay report latencies shorter than
they should; on systems with fast enough drivers, the delay is not
perceptible, but high enough invisible delays would cause desyncs.

I'm not yet completely sure whether this is 100% accurate, there are
some issues involved when repeatedly pausing+unpausing (the delay might
jump around by several dozen miliseconds), but seeking seems to be
working correctly now.
ao_wasapi: Slightly improve timer accuracy

Use QueryPerformanceCounter to improve the accuracy of
IAudioClock::GetPosition.

While this is mainly for "realtime correctness" (usually the delay is a
single sample or less), there are cases where IAudioClock::GetPosition
takes a long time to return from its call (though the documentation doesn't
define what a "long time" is), so correcting its value might be important in
case the documented possible delay happens.
Merge pull request #619 from mpv-player/wasapi/better_timing

ao_wasapi: Add device latency to get_delay