const MAX_TILE_LEVEL: 23
The deepest region level a tile frame is exact at. A face coordinate at
level n is a multiple of 2⁻ⁿ, and writeTileFrame carries the corner
and the sample step as float32 fractions of the cell; past 23 the step is
below the mantissa and the frame would round. A source that produces tiles
from the kernel says so through HeightfieldSource.maxLevel, and the
streamer sends anything deeper to the pool.