func makeBall(b ball) Ball {
img := canvas.Box(ballSize, ballSize, image.NewUniform(b.col), 1, image.Black)
p := b.p.point().Sub(image.Pt(ballSize/2, ballSize/2))
item := canvas.NewImage(img, true, p)
window.AddItem(item)
window.Raise(item, nil, false)
return Ball{item}
}
func makeRect(r image.Rectangle, col color.Color) {
img := canvas.Box(r.Dx(), r.Dy(), image.NewUniform(col), 1, image.Black)
item := canvas.NewImage(img, opaqueColor(col), r.Min)
window.AddItem(canvas.Draggable(item))
}