Performance: Cache NPC goal

[dseguin]

Summary

None

Purpose of change

While testing NPCs in #54307 I noticed the game was running pretty slow:

Describe the solution

It looks like overmapbuffer::find_closest is a little too expensive to be running every turn. I decided to cache this value, and refresh it when the NPC moves between OMTs and on game load. In-game, the turn speed went from ~5 seconds/turn to ~2 turns/second (x10 improvement). This is compiled with all debug flags and NOOPT=1.

perf shows a pretty significant improvement:

Describe alternatives you've considered

I guess npc::long_term_goal_action could be delayed for X turns before being called again (not sure about the repercussions of that).

Testing

Stepping through npc::set_omt_destination, the goal is cached for each need. Moving OMTs forces the goal to be recalculated (which is noticeable in-game, being a bit slower). The location candidates for each need don't change anyway, so there's no benefit from running this check every time.

Additional context

This is mostly a problem for NPCs without a set destination (which is most NPCs, apparently).