Fix an active item cache bug that occurred upon map shift

[Davi-DeGanne]

Summary

SUMMARY: Bugfixes "Fix an active item cache bug that occurred upon map shift"

Purpose of change

The cache of submaps with active items was being cleared too liberally in map::shift. Two "edges" were always being cleared (e.g. both north and east edges), even if the shift was occurring only in one direction (e.g. south, but not west or east). This means that sometimes, loaded submaps on the edge of the reality bubble with active items were being dropped from the cache, making the game "forget" about their items.

Partially fixes #32936
Partially fixes #33187

There are/were other things causing the same bug, but this was certainly a major contributing factor.

Describe the solution

Simply check that we are shifting away from an edge before dropping that edge's submaps from the cache.

Describe alternatives you've considered

Testing

@kevingranade confirmed that he had a reproduction case that this change successfully fixed.

Additional context

I fixed a blocker, wooo!
Actually this was mostly kevin's and ralree's work, I just swooped in at the last minute and stole all the credit 😁