Replace SHA1 with adler32. For simple purposes of checking to see if players are playing the same map, this should be quite adequate and runs 15 times faster.
This reduces CheckLand ~5.5% on average over prior making the overall reduction ~77.4% instead of ~81.9%. It does skip centre pixel in odd w/h, but that really shouldn't matter much in this case. Can alter if any objects are noticeably off.