title: GPUImage crash ('over release framebuffer') when using Blend Filter
body: <p>I'm getting a headache because of the crash I experience when trying to apply Blend filter to an image and displaying it.
  What I want to do is simply put an overlay image onto another image.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>- (GPUImageOutput&lt;GPUImageInput&gt; *)myFilter
  {
      GPUImageFilterGroup *filtersGroup = [GPUImageFilterGroup new];
  
      // Saturation
      GPUImageSaturationFilter *saturationFilter = [GPUImageSaturationFilter new];
      saturationFilter.saturation = 0.0;
      [filtersGroup addFilter:saturationFilter];
  
      // Noise
      UIImage *noiseImage = [UIImage imageNamed:@"noise.png"];
      GPUImagePicture *noisePicture = [[GPUImagePicture alloc] initWithImage:noiseImage];
      GPUImageAddBlendFilter *blend = [GPUImageAddBlendFilter new];
      [blend useNextFrameForImageCapture];
      [filtersGroup addFilter:blend];
      [noisePicture addTarget:blend atTextureLocation:1];
      [noisePicture processImage];
  
      [saturationFilter addTarget:blend];
  
      filtersGroup.initialFilters = @[saturationFilter];
      filtersGroup.terminalFilter = blend;
  
      return filtersGroup;
  }
  </code></pre>
  
  <hr>
  
  <hr>
  
  <pre><code>    // Applying filter
      GPUImageOutput &lt;GPUImageInput&gt; *effect = [self myFilter];
      self._photoHandle = [[GPUImagePicture alloc] initWithImage:staticImage];
      [self._photoHandle addTarget:effect];
      [effect addTarget:self.targetPreviewView];
      [self._photoHandle processImage]; // Crash
  </code></pre>
  
  <p>When I try to run it, app crashes and I get this message:</p>
  
  <blockquote>
    <p>Terminating app due to uncaught exception
    'NSInternalInconsistencyException', reason: 'Tried to overrelease a
    framebuffer, did you forget to call -useNextFrameForImageCapture
    before using -imageFromCurrentFramebuffer?'</p>
  </blockquote>
  
  <p>I do call 'useNextFrameForImageCapture' on blend filter so what's wrong with my code?</p>