added missing semicolon from removing filterLength class var
made Mixer3D::inputAO local to Mixer3D::addAudioObjToMix

also renamed inputAO to audioObjInput
fixed typo in comment
made overlapInput a 2D array

previously it was only a 1D array, meaning when the Mixer was
single-threaded the 'overlapInput' from the previous iteration, for
ALL audio objects, would actually just be the overlapInput from the
previous iteration for the LAST audio object, since at each iteration
across the audio objects, the overlapInput array would get re-written.
remove whitespace