Greatly improve terrain_tilemap_calc_next().
Greatly improve terrain_tilemap_calc_next().

- No longer mucks around with uninitialised memory for temporary
  results.

- Substantially faster since it doesn't recalculate the lower tile 4
  times for every upper tile.

Load time: 7.3s -> 6.2s

AltSysrq/mantigraphia,AltSysrq/mantigraphia