constructor() {
    this.startClientPoint = null;
    this.startTimestamp = (new Date()).valueOf();
    this.deferredPoints = [];
  }
This state is active when the user has initiated the drawing action but has
not yet created any lines. We use this state to try and detect a second touch
event to initiate a pinch-zoom action. We'll give up on that if enough time
or movement happens without a second touch.