- Use the SDL_AUDIO_MASK_DATATYPE bit when selecting an implementation
where it matters. Previously two existing AUDIO_F32 cases had been
written, but were unreachable.
- Add AUDIO_F32 case for SDL_ConvertSurround_4.
- Fix incorrect pointer arithmetic causing the 2 to 6 channel
conversion for 4 byte audio formats to read and write beyond the end
of the buffer.
