regionAt
Computes the region this pan shows at time, as a fraction of the frame entering it.
Every backend that draws a pan reads its region from here, so every engine puts the same clip in the same place at the same composition time.
time is composition-relative on every backend, so span is the clip's own slot on the composition timeline. A time before the span holds KenBurns.from and a time after it holds KenBurns.to, so a frame landing a rounding error outside the slot draws the end it is nearest.
Return
The region that fills the frame at time.
Parameters
time
Where the frame sits on the composition's timeline.
span
The clip's slot on that timeline.