provide renderer function that takes L and R buffer arguments separately

[atsushieno]

I have been building an audio plugin project that builds VST/AU/LV2/CLAP plugins using TinySoundFont. It is in general working well, but one thing I find some room for improvements is that tsf_render_float() cannot handle audio output buffers in float**.

If the audio plugin output buffers are passed as float** and outputs[0] and outputs[1] are NOT consecutive, then it cannot be passed as float* as in tsf_render_float() and it will involve extraneous buffer copying.

It is quite easy to rewrite tsf_render_float() to take two distinct buffers so it would be nice if TSF API also offers such an overload(-ish) renderer function.

I have a patch that makes it.

TSF commit @ cd82859