The superclock rate is variable and is stored in session files since
7.0 . It is set globally when reading from a session file, and it really
should be reset to a known state whenever a new blank session is
created from an existing one. Currently, that doesn't happen. We fix
that by resetting it in Temporal::reset().
For now, we are duplicating the default superclock rate 282240000 from
superclock.h , but we can drop that when all code paths that use
superclock also use Temporal::reset().
This will provide an (extra) guarantee that
set_superclock_ticks_per_second() always is invoked before creating
TempoMap or using superclock_ticks_per_second() in other ways. The
DEBUG_EARLY_SCTS_USE in superclock.h is thus closer to passing.
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