lnw/space-filling-curve-layout

2d space filling curve layouts for std::mdspan

Experimental 2D space filling curve layouts for std::mdspan

So far there are 2 space filling curves implemented: Hilbert (with blocks of 256x256) and Morton (with blocks of 256x256 or 16kx16k). For either of them the layout places squares in row major order; an image type making use of them sacrifices memory for speed if the requested image size is not a multiple of the block size. Larger blocks may offer better cache line utilization (with quickly diminishing returns) at the expense of more wasted memory and more expensive index computations.