title: Rotate object: How to start rotation by the last radian value?
body: <p>I have this function which I like to extend, so the current rotation of an image will get considered.</p>
  <p>The current behavior is that whenever I start to rotate the image by the mouse-cursor, it resets first to 0 rad, so if the image is rotated to 90 degrees first, it will jump to 0 on the next attempt to rotate the image.</p>
  <p>I want it to start from the last radian value. I'm bad at math, would be nice if someone could help me out on this one.</p>
  <pre><code>Coordinate = [x,y]
  Center = [x,y]
  Extent = [minX, minY, maxX, maxY]
  
  private get angleDelta(): number {
  
      if (this.isDragging) {
          return 0;
      }
  
      const extent = this.object.getExtent();
      const cursor = this.getCursorCoordinates();
  
      if (!extent || !cursor) {
          return 0;
      }
  
      const center = getCenter(extent);
  
      return getAngle(center, cursor) - getAngle(center, this.startCoordinate)
  }
  
  
  function getAngle(start: Coordinate, end: Coordinate): number {
      return Math.atan2(start[1] - end[1], start[0] - end[0]);
  }
  </code></pre>