wasapi: Allow opening devices in exclusive mode

wasapi: Allow opening devices in exclusive mode

This provides much lower latency compared to opening in shared mode,
but it also means that the device cannot be opened by any other
application. The advantage is that the achievable latency is much
lower.

In shared mode, WASAPI's engine period is 10ms, and so that is the
lowest latency achievable.

In exclusive mode, the limit is the device period itself, which in my
testing with USB DACs, on-board PCI sound-cards, and HDMI cards is
between 2ms and 3.33ms.

We set our audioringbuffer limits to match the device, so the
achievable sink latency is 6-9ms. Further improvements can be made if
needed.

https://bugzilla.gnome.org/show_bug.cgi?id=793289