common: Add a user_data pointer to audio_mixer_sound

[RobLoach]

This change adds a void* userdata to the audio_mixer_sound_t struct. This allows you to keep some context of the sound that's being stopped when using the audio_mixer_stop_cb_t stop_cb callback in audio_mixer_play().

Without this, you need to reference globals within the thread to retrieve any kind of context, which can be problematic, and not thread-safe.

Example

In the below example, you can see we pass an AppContext pointer which is thread-safe and doesn't mess with globals.

void sound_finished(audio_mixer_sound_t* sound, unsigned reason) {
  AppContext* app = sound->user_data;
}

// ...

mySound->user_data = app;
audio_mixer_play(mySound, false, 1.0f, NULL, 0, sound_finished);