oordinates of the mouse position
    function mousePos(e) {
        var rect = canvas.getBoundingClientRect();
        return new mapboxgl.Point(
            e.clientX - rect.left - canvas.clientLeft,
            e.clientY - rect.top - canvas.clientTop
        );
    }

    function mouseDown(e) {
        // Continue the rest of the function if the shiftkey is pressed.
        if (!(e.shiftKey && e.button === 0)) return;

        // Disable default drag zooming when the shift key is held down.
        map.d