The inverted atomic flag replaces test_and_set+clear with test_and_clear+set,
essentially inverting the flag status. This makes more logical sense for
flagging dirty state, which is less confusing than flagging clean state. The
one caveat is ATOMIC_FLAG_INIT (or default construction in C++20) initializes
the state to true rather than false.
No other effect depends on context or listener properties, so reverb being the
only exception for speed of sound and meters per unit was putting extra work on
the effect engine for no real reason. Especially since the reverb decay time
should be the time actual time to decay irrespective of other settings.
