const LEVEL_DRAW_AT: 36
Where each rung's existence threshold sits in the words, one u32 per rung
in walk order, on a uvec4 boundary past WORD.
levelContribution places a crater where toUnit(hash.x) < density, and
toUnit is hash / 2³² exactly in float64 — so the test is hash < density · 2³² in real arithmetic, which for an integer hash is
hash <= ceil(density · 2³²) − 1. That last number is what is stored, and
the kernel compares integers. Taken in float32 instead, a hash within one
part in ten million of the line lands on a different side of it than the
CPU put it, and that is a crater that exists on one processor and not the
other.