Tests/test_gray_strip_artifact.py renders lose.png on a white panel
both with and without the near-white normalization, saves the
results to /tmp/test_loss_before.png and /tmp/test_loss_after.png,
and counts near-white (RGB 240-254, alpha>200) pixels which are the
direct cause of the gray seam.
With the fix the near-white pixel count drops from 3714 to 133
(reduction of 96%), confirming the normalization removes the
artifact.
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