updateActivePointers_(pointerEvent) {
    const event = pointerEvent;
    const id = event.pointerId;

    if (
      event.type == MapBrowserEventType.POINTERUP ||
      event.type == MapBrowserEventType.POINTERCANCEL
    ) {
      delete this.trackedTouches_[id];
      for (const pointerId in this.trackedTouches_) {
        if (this.trackedTouches_[pointerId].target !== event.target) {
          // Some platforms assign a new pointerId when the target changes.
          // If this happens, delete one tracked pointer. If there is more
          // than one tracked pointer for the old target, it will be cleared
          // by subsequent POINTERUP events from other pointers.
          delete this.trackedTouches_[pointerId];
          break;
        }
      }
    } else if (
      event.type == MapBrowserEventType.POINTERDOWN ||
      event.type == MapBrowserEventType.POINTERMOVE
    ) {
      this.trackedTouches_[id] = event;
    }
    this.activePointers_ = Object.values(this.trackedTouches_);
  }
Keeps track on how many pointers are currently active.

@param {PointerEvent} pointerEvent Pointer
event.
@private