Fix some VC6/2003 errors

This commit fixes an issue where the optimized mono expansion path is
never hit.

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

VC6 doesn't appear to have SetFilePointerEX, so fallback on
SetFilePointer instead. This matches-up with VC6 not supporting
64-bit fseek.

Visual Studio .NET 2003 supports it, however.

For whatever reason, neither compiler likes implicit casts between
`volatile unsigned int*` and `int*`.

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.