title: GPUImage blend with self in filter chain
body: <p>I am trying to create a filter chain where one step takes the output and blends it with itself. When I try to do this I get the error "Incomplete filter FBO: 36054". What do I need to do to make this blend use the output of a previous filter for both inputs?</p>
  
  <pre><code>+ (UIImage *)filter:(UIImage*)image {
  
      GPUImagePicture *picture = [[GPUImagePicture alloc] initWithImage:image];
  
      // SETUP FILTERS
      GPUImageBilateralFilter *bilateralFilter = [[GPUImageBilateralFilter alloc] init];
      bilateralFilter.texelSpacingMultiplier = 2;
      bilateralFilter.distanceNormalizationFactor = 10;
  
      GPUImageSoftLightBlendFilter *softLightBlend = [[GPUImageSoftLightBlendFilter alloc] init];
  
      GPUImageNormalBlendWithAlphaFilter *masterBlend = [[GPUImageNormalBlendWithAlphaFilter alloc] init];
      masterBlend.alpha = 0;
  
      // SETUP PIPELINE
      [picture addTarget:bilateralFilter];
      [bilateralFilter addTarget:softLightBlend];
  #if true
      [bilateralFilter addTarget:softLightBlend]; // doesn't work
  #else
      [picture addTarget:softLightBlend]; // works
  #endif
  
      [softLightBlend addTarget:masterBlend];
      [picture addTarget:masterBlend];
  
      // PROCESS PIPELINE
      [picture processImage];
  
      UIImage *result = [masterBlend imageFromCurrentlyProcessedOutputWithOrientation:UIImageOrientationUp];
      return result;
  }
  </code></pre>
  
  <p>This simpler version also results in the same error:</p>
  
  <pre><code>+ (UIImage *)test:(UIImage*)image {
  
      GPUImagePicture *picture = [[GPUImagePicture alloc] initWithImage:image];
  
      // SETUP FILTERS
      GPUImageSoftLightBlendFilter *softlightBlend = [[GPUImageSoftLightBlendFilter alloc] init];
  
      GPUImageNormalBlendWithAlphaFilter *masterBlend = [[GPUImageNormalBlendWithAlphaFilter alloc] init];
      masterBlend.alpha = .5;
  
      // SETUP PIPELINE
      [picture addTarget:softlightBlend];
      [picture addTarget:softlightBlend];
  
      [softlightBlend addTarget:masterBlend];
      [picture addTarget:masterBlend];
  
      // PROCESS PIPELINE
      [picture processImage];
  
      UIImage *result = [masterBlend imageFromCurrentlyProcessedOutputWithOrientation:UIImageOrientationUp];
      return result;
  }
  </code></pre>