Rewrite handling of DTK (streaming) audio.

The primary motivation here is to make sure we submit samples from the
CPU thread. This makes sure the timing of related interrupts accurate,
and generally keeps the different kinds of audio synchronized.  This will also
allow improvements to audio dumping functionality.

The new code is also more concise because it gets rid of some duplicated
audio mixing code.
Audio mixer: remove check for ratio > 1.0.

The code actually handles this case correctly; the algorithm is linear
interpolation between the two closest samples, and the way it is written
should work correctly with any ratio.
Audio mixer: fix a couple compiler warnings.
Streaming/DTK audio: fix pausing playback.
Try to make DTK rewrite handle events the same way as the old code.
AudioCommon: restore support for DTK volume setting.
Merge pull request #259 from magumagu/dtk-rewrite

DTK rewrite