Merge all audio output locks except volume control
With only one input per output, the lock is only useful: - to restart the audio instance (needed 3 out of 4 locks anyway), - to propagate volume change to the mixer or output plugin, - to access the FIFO from the output plugin thread (if applicable). So 4 fine-grained was over-engineering. Most importantly, the locking scheme was overly complicated and generally misunderstood/misused. Also to avoid lock inversion, some unlocking/relocking sequences were introduced; they broke atomicity. We could certainly reduce the scope of the remaining lock. Since we have one only input per output, most of the code is only ever run from the decoder thread. Thus reentrancy is not anymore needed in some places. But first aout_Restart() needs to be fixed and simplified.
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