Function draw_filmstrip

fn draw_filmstrip(
    p: &Painter,
    ectx: &Context,
    state: &TimelineState,
    clip: &Clip,
    asset: &Asset,
    rect: Rect,
    vis: Rect,
    thumbs: &mut ThumbCache,
)

Filmstrip of source-time thumbnails across a video/image clip. Non-blocking: misses are queued in the ThumbCache worker and painted on a later frame.