fix: fix timing calculation when you have offsets

Explanation of changes

Something funky happens when you plot a schedule that contains a voltage offset and the schedule has repetitions

schedule = Schedule("hello", repetitions=2)
schedule.add(IdlePulse(duration=4e-9))
# schedule.add(VoltageOffset(1, 0, port=qubit.ports.gate()))
schedule.add(SquarePulse(amp=1, duration=4e-9, port=qubit.ports.gate()), rel_time=96e-9)

the second image shows when we include a voltage offset, but then the total plot duration also increases (because of repetitions=2).

This change fixes it. I hope.

Motivation of changes

Motivate why the particular solution was chosen.

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