Fix uninitialized alpha channel in classic Kuwahara filter

Was only happening for the tiled compositor implementation.

This is likely to be the source of flackey tests on the
buildbot.

Also, the reference image was empty because of this, so
it needs to be re-generated.

Pull Request: https://projects.blender.org/blender/blender/pulls/108800

source/blender/compositor/operations/COM_KuwaharaClassicOperation.cc

@@ -101,6 +101,11 @@ void KuwaharaClassicOperation::execute_pixel_sampled(float output[4],
     }
     output[ch] = mean[min_index];
   }
+
+  /* No changes for alpha channel. */
+  float tmp[4];
+  image_reader_->read_sampled(tmp, x, y, sampler);
+  output[3] = tmp[3];
 }
 
 void KuwaharaClassicOperation::set_kernel_size(int kernel_size)