every write pass deletes existing tracks, which means it also deletes any existing
EOT event. Rather than try to replicate the _length value() that is kept in a
Source object in the SMF object, add a virtual method to SMF that returns
the _length value (or std::numeric_limits<Beats>::max() if not set).
If the _length value is not the max, we will add EOT events to each track
(usually just one) right before writing to disk.
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