More work on the new engine API.

* 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.

research/ma_engine.c

@@ -34,6 +34,7 @@ int main(int argc, char** argv)
             return (int)result;
         }
 
+        ma_engine_sound_set_pitch(&engine, &sound1, 0.75f);
         ma_engine_sound_set_looping(&engine, &sound1, MA_TRUE);
         ma_engine_sound_start(&engine, &sound1);