move direct sound specific behavior into the actual direct sound function

I'd really like to get rid of the `#ifdef _WIN32` there, since it doesn't really gel very well with having different behavior for the Windows and Linux QT versions. This changes nothing about the windows version, it just moves around the `numFrames = numSamples;` from the mix function to the `numBytesRendered = numBytesToRender;` line in the `DSound::soundThread` function.