Function shape_group 

pub fn shape_group(shapes: impl IntoIterator<Item = ShapePiece>) -> impl Piece

Flatten many shape descriptions into one canvas leaf: one native view no matter how many shapes (docs/shapes.md §3.6). Shapes draw in order; reactive properties re-record the group. Child gestures are not wired inside a group, so put .on_tap on the group via Decorate.