Audio mixer: hack for dtk playback speed.

Make sure dtk playback doesn't deliver more samples than it should.  The
fact that Callback_GetStreaming is called from the sound thread in the
first place is a hack, and this will only work properly on the OpenAL
backend because it doesn't account for the mixer's framelimit adjustments,
but this is slightly less ugly than the OpenAL backend trying to guess how
many samples the CPU thread has delivered.