This causes the thread to only start when the IMediaFilter::Run/Pause
calls have been made, and stop whenever either the IMediaFilter::Stop
call has been made, or on destruction, whichever comes first.
This potentially will work around a suspected race condition that
appears to be in the WebRTC library where the filter's library will be
released while the filter is in the process of being destroyed, which
can take longer than usual if the join takes too long. Basically, fixes
a reported crash (that doesn't appear to technically be our fault) when
the filter is used with browsers when the virtualcam is deactivating in
web browsers.
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