Function paint_group 

pub fn paint_group<P: Pixel>(
    parent: &mut Bitmap<P>,
    group: GroupBitmap<P>,
    pipe: &PipeState<'_>,
) -> Clip

Composite a finished group back into the parent bitmap and return the saved clip.

pipe controls blend mode, opacity, and transfer for the compositing step. The group’s own alpha plane is folded into the effective source alpha as eff_a = div255(group_alpha × pipe.a_input).

Pixels with group_alpha == 0 are skipped entirely (no-op fast path).

After this call group is consumed and its bitmap is dropped.