This is happening because the data buffer is incorrectly being reported
as busy (still loading) which is used to indicate to the engine that no
data is available and therefore nothing can be played.

This was always returning the input frame count rather than the number
of frames processed.

Public issue https://github.com/dr-soft/miniaudio/issues/191

This is useful for enabling or disabling fading around loop
transitions.

With this commit, you can now pass in a notification object which is
fired when an asynchronous sound has finished loading.

Previously you could set the group to NULL in which case the master
group would be used, but this has now changed and the group parameter
can never be NULL. Use ma_engine_get_master_sound_group() to retrieve
the master sound group.

  * ma_engine_sound_*() renamed to ma_sound_*().
  * ma_engine_sound_group_*() renamed to ma_sound_group_*().

  * Removed ma_engine_sound_set_fade_in/out()
  * Add ma_engine_sound_set_fade_point_in_frames()
  * Add ma_engine_sound_set_fade_point_in_milliseconds()
  * Add ma_engine_sound_set_stop_delay()
  * Add ma_engine_sound_get_time_in_frames()
  * Removed ma_engine_sound_group_set_fade_in/out()
  * Add ma_engine_sound_group_set_fade_point_in_frames()
  * Add ma_engine_sound_group_set_fade_point_in_milliseconds()
  * Add ma_engine_sound_group_set_stop_delay()
  * Add ma_engine_sound_group_get_time_in_frames()

The fade in/out system has been replaced with something more general
and flexible which allows for up to two fade points to be configured
per sound or group, with arbitrary time periods and volumes.

This commit also includes the addition of a placeholder parameter for
ma_engine_sound_init_from_file() which is used to notify the caller
when an asynchronously loaded sound has finished loading.

Fix logic that controls whether dr_wav is included

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).