Mix 16-bits PCM instead of fixed-point if input is 16-bits or less
Audio output should be more efficient on FPU-less devices. Firstly the mixer should be faster with 16-bits instead of 32. Secondly FPU-less systems usually output at 16-bits, so one sample format conversion between the mixer and the output is now avoided. Thirdly conversion from input format to the mixer format should similarly be completely avoided or at least accelerated.