Where a patch starts and finishes sliding onto its parent's grid, as
multiples of the distance its own cells subtend cellPixels at.
A patch is selected from that distance outward and its parent takes over at
twice it, so the whole band is [1, 2]. Below MORPH_START a patch draws its
own tessellation; above MORPH_END it is exactly its parent's, which is what
makes the handover invisible.
MORPH_END is 1.8 rather than 2 because the band has to close before the
parent's range rather than at it. A patch is selected from its own range
outward, so a coarser neighbor that did not refine is at least its own range
away — which is exactly twice this patch's, since regionSpacing is nominal
per level and therefore the same everywhere on the cube. 1.8 leaves a tenth
of the band as margin for the eye's own motion between the frame the
selection was taken on and the frame it is drawn on, and costs a sliver of
detail at the far end of a patch's life.