* Initial work on infrastructure for spatialization, panning and
pitch shifting.
* Add ma_engine_sound_set_pitch()
Spatialization and panning is not yet implemented, but pitch shifting
should now be working.
* The engine will now auto-start by default. This can be changed in
the config by setting `noAutoStart` to true.
* Initial implementation of ma_engine_play_sound() which can be used
for fire-and-forget playback of sounds.
* Add ma_engine_sound_at_end() for querying whether or not a sound
has reached the end. The at-end flag is set atomically and
locklessly in the mixing thread.
David Reid