Fetches every tile of a MercTileBlock and composites them into one sRGB
texture (cols·256 × rows·256). Tiles are placed by copyExternalImageToTexture
into their grid cell; a failed tile leaves its cell zero-cleared (rare at valid
zooms). Throws only when every tile failed, so the caller can fall back to flat
coloring (mirrors the single-tile path).
Fetches every tile of a MercTileBlock and composites them into one sRGB texture (
cols·256 × rows·256). Tiles are placed bycopyExternalImageToTextureinto their grid cell; a failed tile leaves its cell zero-cleared (rare at valid zooms). Throws only when every tile failed, so the caller can fall back to flat coloring (mirrors the single-tile path).