Based on some quick* testing using z-order instead of going row by row ends up taking 1-10% less space. Most images were on the lower end but I didn't run into

Based on some quick* testing using z-order instead of going row by row ends up taking 1-10% less space. Most images were on the lower end but I didn't run into any taking more space. * Swapping to z-order requires about 5 lines of changes if you ignore all the little edge cases like non power-of-two image sizes. Images used in testing may or may not be representative of anything.