fn block_grid_lengths(&self) -> [usize; 3] {
        [
            self.block_descriptor_length(),
            self.blocks_wide,
            self.blocks_high,
        ]
    }
Dimensions of a grid of block descriptors, viewed as a 3d array.
Innermost dimension is block descriptor position, then horizontal block location,
then vertical block location.
src/hog.rs