How wide a scatter region is, meters — the ground one r: address covers.
Not a free choice: it is SCATTER_SLOTS slots at SCATTER_SPACING apiece,
written out so that changing either moves the region with it rather than
silently changing the density. 1,024 slots at one per 64 m² is a 256 m square.
The size that results is also the one the renderer wants. Smaller regions mean more of them near the camera and more addresses to reconcile per frame; larger ones mean a region is generated long before any of its rocks is close enough to see, and generating one is a field sample per candidate. At 256 m, the 212 m a rock stops being two pixels at is four to nine regions.
It is a target: levelForSize rounds to the nearest level, so the real
figure runs from 0.7 to 1.4 of it and the density with it.