Loads the imagery mosaic for a terrain tile's rectangle, falling back to
progressively COARSER Mercator zooms when the deepest block can't be fetched
(typically Esri 404s for areas it doesn't cover to the deepest level). This is the
imagery-pyramid equivalent of CesiumJS draping a ready ancestor imagery tile while
the exact-level one is unavailable: rather than dropping the tile to the flat
elevation ramp, it keeps real aerial imagery at a lower resolution. Returns the
texture together with the block it was actually loaded at (so the terrain vertex
UVs map into the right block), or null only when every level — down to the
world-spanning z0 — failed (genuine network outage), where the caller falls back to
the elevation ramp.
Loads the imagery mosaic for a terrain tile's rectangle, falling back to progressively COARSER Mercator zooms when the deepest block can't be fetched (typically Esri 404s for areas it doesn't cover to the deepest level). This is the imagery-pyramid equivalent of CesiumJS draping a ready ancestor imagery tile while the exact-level one is unavailable: rather than dropping the tile to the flat elevation ramp, it keeps real aerial imagery at a lower resolution. Returns the texture together with the block it was actually loaded at (so the terrain vertex UVs map into the right block), or null only when every level — down to the world-spanning z0 — failed (genuine network outage), where the caller falls back to the elevation ramp.