WASAPI device volume changes doesn't propagate back to libsoundio

The volume for WASAPI devices are currently retrieved only when the device is opened. If someone decides to change the application volume via OS volume mixer, the volume value does not get updated in SoundIoOutStream structure.

With raw WASAPI streams, the newly added volume interface does not change the stream volume at all. Raw/exclusive streams (and maybe shared streams too?) should use the [IAudioEndpointVolume](https://msdn.microsoft.com/en-us/library/windows/desktop/dd370832(v=vs.85).aspx) interface over ISimpleAudioVolume for controlling volume and receiving volume notifications.