* 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.