Allow keeping handles on SoundEffect

[ikskuh]

Add a retain-release-logic to SoundEffect so it is possible to keep handles to played sounds and modify stuff like volume or panning while the sound plays.

basic idea is that there's a atomic reference counter and release() requires a EventPool where the sound is freed with when the refcount goes to zero.

const handle = try audio.playSound(sound, .{
  .keep_reference = true, // this is opt-in behaviour, we would not retain the sound handle by defaul
});
…
handle.pan += 0.2 * delta_time;
…
audio.releaseSound(handle);

Expose the following features to the user:

• is_playing
• pan
• volume