On macOS, apps are supposed to play UI-related sounds using the "system
default" device. This commit creates a new instance of BGM_Driver, which
BGMApp sets as the system default device. BGMApp ignores audio played to
that device when deciding whether to pause/unpause the user's music
player.
Since UI sounds are short, this helps avoid pausing the music player and
unpausing it shortly after.
It turns out that the HAL will sometimes call BGM_Driver::StartIO before
sending kAudioDevicePropertyDeviceIsRunning to BGMPlayThrough. In that
case, BGMApp would start playthrough and tell BGMDriver to return from
StartIO immediately, which meant we would drop the initial frames while
the output device started up.
So now BGMApp waits for the output device in that case as well.
Fixes#7.
