A Mercator tile block covering the rectangle at a specific zoom z (vs
mercatorBlockForRect, which picks the deepest zoom ≤ MOSAIC_MAX_PER_AXIS
per axis). Used by loadImageryWithFallback to step to a coarser, more
widely-available level when the deepest block 404s. Coarser zooms cover the rect in
fewer tiles, so the per-axis count only shrinks.
A Mercator tile block covering the rectangle at a specific zoom
z(vs mercatorBlockForRect, which picks the deepest zoom ≤MOSAIC_MAX_PER_AXISper axis). Used by loadImageryWithFallback to step to a coarser, more widely-available level when the deepest block 404s. Coarser zooms cover the rect in fewer tiles, so the per-axis count only shrinks.