[Linux/SDL2] Audio volume settings are not saved properly

[vanfanel]

Hi there,

At least using the SDL2 audio backend on GNU/Linux, changing the volume settings in Settings->Audio and quitting the game, some of the settings are not saved.
Sometimes, the Master volume value is saved, but the rest of the values are set to 99% upon running the game again.

Possibly related to:
#165

Again with this, I suspect a variable overflow in the audio volume management.