A tile's destination rectangle was placed at x0 * scale, and that scale
is rarely whole, so every 256px boundary landed on a fraction of a pixel.
The edge was drawn half covered, stayed transparent, and the JPEG export
flattened that transparency onto black: a dark line down each seam.
Snap both destination edges to whole pixels instead, so neighbouring
tiles share the exact same boundary, and make the destination context
opaque so no partly covered pixel can survive as transparency again.
Measured on a 640px source: the seam at 2K was 46 levels darker than its
neighbours (96 at 4K); it is now within one level of them.
3dtours
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