Adds a source which tests whether audio buffering affects audio sync.
This sync test cycles through 7 audio/video frequencies at 250
millisecond intervals, and via a hotkey, will artificially move audio
time back by one second (and the audio cycle back by 4 frequencies).
This will artificially increase audio buffering by approximately 750
milliseconds.
Results from this test as of this writing: this test proves that dynamic
audio buffering does not affect sync.
jp9000