Fading is now set using these APIs:

  * ma_engine_sound_set_fade_in()
  * ma_engine_sound_set_fade_out()

When a sound is stopped, either by naturally reaching the end, or
explicitly with ma_engine_sound_stop(), the fade out will be applied.
Fading will also be applied around loop transitions.

Note that when a sound is stopped implicitly by it reaching the end,
fading out will not work when the length of the sound is not know (that
is, when ma_data_source_get_length_in_pcm_frames() returns 0).