fn video_dirty_spans(old: &Project, new: &Project) -> Option<Vec<(f64, f64)>>Expand description
Timeline spans (seconds) whose cached frames the edit old -> new can have changed.
None = the change can affect any frame (or is too entangled to bound): clear everything.
Edits that cannot change pixels (markers, in/out points, audio tracks, buses, planner, notes,
labels, folders) produce no spans at all — the whole cache survives them.