_initBaseEvent(event, pointer) {
    const properties = {};

    properties["pointer"] = {
      DOM: { x: pointer.x, y: pointer.y },
      canvas: this.canvas.DOMtoCanvas(pointer),
    };
    properties["event"] = event;

    return properties;
  }
Create an object containing the standard fields for an event.
@param {Event} event
@param {{x: number, y: number}} pointer Object with the x and y screen coordinates of the mouse
@returns {{}}
@private