AuxTimeline
How long an AuxInput's branch runs for, and at what rate it advances.
A branch fed by a single still ends on its first frame, which leaves a time-varying filter on it with nothing to run on and lets the merge downstream stop drawing it. Repeating the still at frameRate for duration keeps the branch alive for as long as the one it merges onto.
duration is counted from the branch's own start rather than from the composition's, so a clip effect names its clip's length and a composition effect names the whole composition's.