title: Issue scaling layer at the center of a pinch gesture
body: <p>I am currently having a map(tilemap) within a layer that i would like to pan/zoom using the following code: </p>
  
  <pre><code>- (void) pinchGestureUpdated: (UIPinchGestureRecognizer *) recognizer {
  
      if([recognizer state] == UIGestureRecognizerStateBegan) {
  
              _lastScale = [recognizer scale];
  
              CGPoint touchLocation1 = [recognizer locationOfTouch:0 inView:recognizer.view];
              CGPoint touchLocation2 = [recognizer locationOfTouch:1 inView:recognizer.view];
  
              CGPoint centerGL = [[CCDirector sharedDirector] convertToGL: ccpMidpoint(touchLocation1, touchLocation2)];
              _pinchCenter = [self convertToNodeSpace:centerGL];
      }
  
      else if ([recognizer state] == UIGestureRecognizerStateChanged) {
  
  //        NSLog(@"%d", recognizer.scale);
  
          CGFloat newDeltaScale = 1 -  (_lastScale - [recognizer scale]);
          newDeltaScale = MIN(newDeltaScale, kMaxScale / self.scale);
          newDeltaScale = MAX(newDeltaScale, kMinScale / self.scale);
  
          CGFloat newScale = self.scale * newDeltaScale;
  
          //self.scale = newScale;
          [self scale: newScale atCenter:_pinchCenter];
  
          _lastScale = [recognizer scale];
      }
  }
  
  - (void) scale: (CGFloat) newScale atCenter: (CGPoint) center {
  
      CGPoint oldCenterPoint = ccp(center.x * self.scale, center.y * self.scale);
  
      // Set the scale.
      self.scale = newScale;
  
      // Get the new center point.
      CGPoint newCenterPoint = ccp(center.x * self.scale, center.y * self.scale);
  
      // Then calculate the delta.
      CGPoint centerPointDelta  = ccpSub(oldCenterPoint, newCenterPoint);
  
      // Now adjust your layer by the delta.
      self.position = ccpAdd(self.position, centerPointDelta);
  }
  </code></pre>
  
  <p>my issue is that the zoom is not taking in effect at the center of the pinch even though i am trying to change it at the same time i am zooming in through this method:  <code>(void) scale: (CGFloat) newScale atCenter: (CGPoint) center</code>. Is there any reason this might not happening properly? Also how do i convert to the center location of the pinch into the coordinates system for my scene/layer?</p>