Function frame_needs 

pub fn frame_needs(
    project: &Project,
    index: &ActiveIndex,
    time: MediaTime,
) -> Result<Vec<FrameNeed>, EvalError>

Describes all source frames needed at a timestamp.

The returned list is sorted in evaluation order and contains no duplicate (source, timestamp) pairs. Generated sources need no frames. A visual clip needs its own source only when it draws it (see ActiveClip::needs_source), and every source its composition’s media steps read, owned by the clip; a matte clip needs them only while an active clip names it (FrameDesc::draws).

Errors

Returns any error emitted by evaluate.