Follow on a clip read the clip's own footage and doubled its motion, the
ffmpeg path ran clip effects before the look and the preview ran them on
the unframed source, splits reset the effect clock and dropped eases,
vignette, flash and light leak painted transparent pixels, and tracked
mask keys ignored trims and speed changes, sampled at 15 fps on ffmpeg,
and slipped when the item moved mid-read.

Follow reads the video under the item, both renderers run the look and
then the effects on the framed picture, splits carry effectsFrom and
redraw eased moves, effects keep transparent edges, and patchClip moves
mask keys with retiming. Keyframe lookups cache their play order.