Function zoom_at 

pub fn zoom_at(
    seg: &ZoomSegment,
    frame: Frame,
    ramp_frames: Frame,
) -> ZoomTransform

The zoom transform at project frame for one segment.

Ramps identity → seg.amount → identity over [start, end) using the segment’s ease. ramp_frames is the desired in/out ramp; it is clamped to half the window so the ramps never overlap. Outside the window (or for an empty window) the result is ZoomTransform::identity.