Custom terrain defs - make them load order indifferent

[RemingtonRyder]

If there are two mods active, each having terrain defs in common (for example, Harsher Tundra and Ice Sheet, before I decided to merge them) then as you would expect, the last version of a particular def loaded in the mod list decides which def is used. That bit is fine.

However, if the load order is changed (perhaps inadvertantly) then saved games which depend on those terrain defs do not load properly - the conflicting terrain is turned into sand instead. Even if the defs are identical.

Basically it means that to avoid this problem, you can only have one combined terrain mod active, or you must always activate the terrain mods in the same order.