ao/wasapi: increase buffer size to 50 ms

Before it was the default device period, which was too small
causing glitches on on entering/exiting fullscreen.
ao/wasapi: periodicity in shared mode must be zero

IAudioClient::Initialize hnsPeriodicity argument is nonzero only for exclusive mode

http://msdn.microsoft.com/en-us/library/windows/desktop/dd370805%28v=vs.85%29.aspx
ao/wasapi: refix printf warning for both cygwin and msys

a cast to (unsigned) should do "the right thing".