aout: remove intricate volume/mute transaction system
Volume and mute states are now independant. There is no need to update them together in a single transaction. Furthermore, other processes can change the volume and/or mute state of VLC playback streams asynchronously. Thus volume-up/volume-down and mute-toggle are not atomic operations even when protected by the volume lock. We would need to have toggle and up/down provided by the to the output plugins. That is probably impossible and overkill. So accept the small race condition and simplify the code.
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