const SLACK: f64 = 1e-6;Expand description
How far under the minimum a tile may come out and still count as at it.
A split’s halves are f64 arithmetic on a box that has already been
through a division or two, so a tile laid out at exactly the minimum can
land a rounding error short of it. A millionth of a pixel is far below
anything drawn and far above that error.