Actual blockmap fix

MI's "fix" was a reversion of something that allowed 2.0 maps to use the entire blockmap. This MR reverts that fix and adds a proper fix to the issue of west/south edges of the blockmap not working as they should. Tested with a thokbarrier-less square map (all sides were solid) and with AGZ (objects are tangible all around the map, like they are in 2.1.14).