Region-gain (unlike other automation) is specific to the
region and independent of the source. Region::start() offset
does not apply. When region-start is trimmed the region's
envelope is modified (not just offset). The event-list is truncated.
Any audio-region envelope does (and must) have a point exactly
at the start and end of the region.
truncate_start() can thus calculate the earliest position of
valid events with the new length relative to the last event.
The mathematical operator for that is subtraction, not distance.
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