function screen_to_canvas(p) {
    // should be called with coordinates obtained from MouseEvent.clientX/clientY * window.devicePixelRatio
    const xc = (p.x - offset.x) / zoom;
    const yc = (p.y - offset.y) / zoom;

    return {'x': xc, 'y': yc};
}

function canvas_to_screen(state, p) {
    const xs = (p.x * zoom + offset.x) / window.devicePixelRatio;
    const ys = (p.y * zoom + offset.y) / window.devicePixelRatio;

    return {'x': xs, 'y': ys};
}