DSound: use DSound notifications to produce sound.

Pretty straightforward; IDirectSoundNotify lets you register for
notifications after a certain amount of sound has played, so use that
instead of depending on Update() notifications from the CPU thread.

Also, while I'm here, reduce the buffer size by a factor of 4; this seems
to reduce the latency, although the difference is sort of subtle.
Merge pull request #256 from magumagu/dsound-notify

DSound: use DSound notifications to produce sound.