Initializing NVAFX is slow as it has to load large models and can cause
significant impact to OBS' startup time. This moves NVAFX initialization
into a separate thread. NVAFX does not appear to be reentrant across
threads, hence a global mutex is also used to avoid reentrancy issues.
The downside of this change is that the first few seconds of audio after
adding the filter or starting OBS will not be filtered, but this is
unlikely to cause any real-world issues.
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