title: GPUImage blend filters
body: <p>I'm trying to apply a blend filters to 2 images.<br>
  I've recently updated <code>GPUImage</code> to the last version.
  To make things simple I've modified the example SimpleImageFilter.<br>
  Here is the code:<br></p>
  
  <pre><code>  UIImage * image1 = [UIImage imageNamed:@"PGSImage_0000.jpg"];
      UIImage * image2 = [UIImage imageNamed:@"PGSImage_0001.jpg"];
      twoinputFilter = [[GPUImageColorBurnBlendFilter alloc] init];
      sourcePicture1 = [[GPUImagePicture alloc] initWithImage:image1 ];
      sourcePicture2 = [[GPUImagePicture alloc] initWithImage:image2 ];
      [sourcePicture1 addTarget:twoinputFilter];
      [sourcePicture1 processImage];
      [sourcePicture2 addTarget:twoinputFilter];
      [sourcePicture2 processImage];
      UIImage * image = [twoinputFilter imageFromCurrentFramebuffer];
  </code></pre>
  
  <p>The image returned is <code>nil</code>.Applying some breakpoints I can see that the filter fails inside the method <code>- (CGImageRef)newCGImageFromCurrentlyProcessedOutput</code> the problem is that the <code>framebufferForOutput</code> is nil.I'm using simulator.<br>
  I don't get why it isn't working.</p>