func TestColorTablesMatch(t *testing.T) {
	const trIdx = 100
	global := color.Palette(palette.Plan9)
	if rgb := global[trIdx].(color.RGBA); rgb.R == 0 && rgb.G == 0 && rgb.B == 0 {
		t.Fatalf("trIdx (%d) is already black", trIdx)
	}

	// Make a copy of the palette, substituting trIdx's slot with transparent,
	// just like decoder.decode.
	local := append(color.Palette(nil), global...)
	local[trIdx] = color.RGBA{}