The previous design had a race condition. When WaveViewThreads::stop_threads() was called, it would
first acquire the mutex, then set _quit, then call condition.broadcast(). But worker threads would
check _quit without holding the mutex. It was therefore for a thread to be delayed in its
own lock acquisition by the ::stop_threads() caller, then end up back in cond.wait() AFTER
the cond.broadcast() was done. Such a thread would sleep forever and never wake up.
This new design removes WaveViewDrawRequestQueue, which was a clean encapsulation of the
queueing aspects of WaveViewThreads, but unfortunately made correct mutex acquisition
and condition signalling/waiting needlessly complex. THe mutex, condition variable
and actual queue were moved into WaveViewThreads, and all worker threads execute a method
of the class which gives the appropriate code easy access to the mutex and condition var,
which must always be used together.
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