title: layer.renderInContext doesn't take layer.mask into account?
body: <p>I'm trying to render some UIImages into 1 single image that I can save in my photo album. But it seems to as if the layer.renderInContext doesn't take a layermask into account?</p>
  
  <p><strong>Current behavior:</strong> the photo saves, and I see mosaicLayer, without the masking effect of maskLayer.</p>
  
  <p><strong>Expected behavior:</strong> the photo saves and I see the image in my view, with on top of that a masked mosaicLayer.</p>
  
  <p>I use the following code to mask the image</p>
  
  <pre><code>UIImage *maskImg = [UIImage imageWithContentsOfFile:[[NSBundle mainBundle]
                          pathForResource:@"mask" ofType:@"png"]];
  
      maskLayer = [[UIImageView alloc] initWithImage:maskImg];
      maskLayer.multipleTouchEnabled = YES;
      maskLayer.userInteractionEnabled = YES;
      UIImageView *mosaicLayer = [[UIImageView alloc] initWithImage:img];
      mosaicLayer.contentMode = UIViewContentModeScaleAspectFill;
  
      mosaicLayer.frame = [imageView bounds]; 
      mosaicLayer.layer.mask = maskLayer.layer;
  
      [imageView addSubview:mosaicLayer];
  </code></pre>
  
  <p>And then i use this code to save my composed image:</p>
  
  <pre><code>UIGraphicsBeginImageContext(imageView.bounds.size);
      [imageView.layer renderInContext:UIGraphicsGetCurrentContext()];
      UIImage *saver = UIGraphicsGetImageFromCurrentImageContext();
      UIGraphicsEndImageContext();
  
      UIImageWriteToSavedPhotosAlbum(saver, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
  </code></pre>
  
  <p><strong>EDIT</strong>: This applies the mask correctly</p>
  
  <pre><code>-(IBAction) saveImage { 
  UIImage * saver = nil;
  CGImageRef image = imageView.image.CGImage;
  
  size_t cWidth = CGImageGetWidth(image);
  size_t cHeight = CGImageGetHeight(image);
  size_t bitsPerComponent = 8; 
  size_t bytesPerRow = 4 * cWidth;
  
  //Now we build a Context with those dimensions.
  CGContextRef context = CGBitmapContextCreate(nil, cWidth, cHeight, bitsPerComponent, bytesPerRow, CGColorSpaceCreateDeviceRGB(), CGImageGetBitmapInfo(image));
  
  //The location where you draw your image on the context is not always the same location you have in your UIView, 
  //this could change and you need to calculate that position according to the scale between you images real size, and the size of the UIImage being show on the UIView. Hence the mod floats...
  CGContextDrawImage(context, CGRectMake(0, 0, cWidth,cHeight), image);
  
  float mod = cWidth/(imageView.frame.size.width);
  float modTwo = cHeight/(imageView.frame.size.height);
  
  //Make the drawing right with coordinate switch
  CGContextTranslateCTM(context, 0, cHeight);
  CGContextScaleCTM(context, 1.0, -1.0);
  
  CGContextClipToMask(context, CGRectMake(maskLayer.frame.origin.x * mod, maskLayer.frame.origin.y * modTwo, maskLayer.frame.size.width * mod,maskLayer.frame.size.height * modTwo), maskLayer.image.CGImage);
  
  //Reverse the coordinate switch
  CGAffineTransform ctm = CGContextGetCTM(context);
  ctm = CGAffineTransformInvert(ctm);
  CGContextConcatCTM(context, ctm);
  
  CGContextDrawImage(context, CGRectMake(0, 0, cWidth,cHeight), mosaicLayer.image.CGImage);
  
  CGImageRef mergeResult  = CGBitmapContextCreateImage(context);
  saver = [[UIImage alloc] initWithCGImage:mergeResult];
  CGContextRelease(context);
  CGImageRelease(mergeResult);
  
  UIImageWriteToSavedPhotosAlbum(saver, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);}
  </code></pre>