Assets like lose.png contain thousands of (254,254,254) pixels on a
background that is also white. When drawn on a white panel these
near-white pixels are slightly darker, producing a visible gray seam.
Apply a global normalization in load_image(): any opaque pixel with
R=G=B >= 250 is clamped to (255,255,255). Applied to every asset, not
only those with transparent_color, so the background is always pure
white regardless of off-by-one in the source PNG.