#853 Unloading tiled maps can kill multitile monsters

[SilverNexus]

When a map is tiled with another map, the unloading of one map when parts of a multitile monster (not the head square, though) are on that map will kill that monster.

I'm guessing this has to do with the fact that the unloaded map makes its position partially off the map, and thus it gets destroyed to prevent segfaults and such.

Attached is a set of maps that demonstates this.

Steps to reproduce:
1. Goto the attached maps (test_multi_left in particular)
2. Aggro the greater demon that starts on the right tile map.
3. Have it chase you toward the left wall (only need to end up a tile or two left of where it started to be patially on the left map like we want)
4. Move to the far left of the map, and wait a few seconds. The demon should die.