Separate music and sound FX volume

[lizzyd710]

Right now setMusicVolume changes all sound volume, not just music, and setSoundEffectsVolume does nothing. It's been like this for a while, I just wanted to create an issue so it wasn't forgotten about.

[lizzyd710]

The relevant bit from AudioMixer.cxx.

void AudioMixer::setMusicVolume(float volume)
{
  // alSourcef(currentSourceID, AL_GAIN, newVolume);
  // this would set the volume, but idk how to get the "currentSourceID"

  // for now set the volume for everything.
  alListenerf(AL_GAIN, volume);
  // update the settings accordingly
  Settings::instance().musicVolume = volume;
}

void AudioMixer::setSoundEffectVolume(float volume)
{
  // find out how to set those volumes seperately
  //alListenerf(AL_GAIN, volume);
  
  // for now, just set the settings value, even if it doesnt do anything
  Settings::instance().soundEffectsVolume = volume;
}

Might have to generate more sources in Soundtrack.cxx, because right now only one is generated, so I think we might need one source for music and one for SFX.