Fast enumerate active items

[kevingranade]

Summary

SUMMARY: Performance "Faster enumeration of active items."

Purpose of change

In profiling while waiting in an empty area of map, 17% of samples occurred in map::process_items().
Upon analyzing the code it became clear that this time was being spent scanning the map for vehicles and active items. This was tolerable when the map was 2D, but now that it's 3D by default, it is way too expensive.

Describe the solution

Use the existing vehicle list cache to enumerate submaps that contain vehicles.
Add a cache of submaps that directly contain active items.

[ifreund]

Looks good to me