Function is_blank_tile 

pub fn is_blank_tile(raster: &Raster) -> bool

Returns true if every pixel in the tile is identical (i.e. the tile is a uniform solid color).

Used by the engine when BlankTileStrategy::Placeholder is active to decide whether a tile should be replaced with a 1-byte marker instead of full image data. A single-pixel raster is trivially blank.

See the blank_tile_strategy tests for integration-level examples.