This adds the following APIs:

  * ma_sound_stop_with_fade_in_pcm_frames()
  * ma_sound_stop_with_fade_in_milliseconds()
  * ma_sound_set_stop_time_with_fade_in_pcm_frames()
  * ma_sound_set_stop_time_with_fade_in_milliseconds()

These functions will overwrite any existing fades. For the
set_stop_time variants, you specify the time that the sound will be put
into it's stopped state. The fade will start at stopTime - fadeLength.
If the fade length is greater than the stop time, the fade length will
be clamped.

Public issue https://github.com/mackron/miniaudio/issues/669