About the pull request

The problem in question is described in this report: cmss13-devs/cmss13#4777

First issue is the interior spaces were not being detected as interiors. For instance, deployables were supposed to be blocked from deploying in interiors. They were being blocked, but not because it was an interior: they were blocked by the turfs themselves being marked as not buildable. Instead of the It's too cramped in here to deploy ... message, you'd get You can't set up ... here. The cause was the interior subsystem populating the used_turfs with weakrefs to a specific type, then in_interior() checking if that list returned that specific type. It didn't, because weakrefs to a type aren't that type: old
Fixed by checking for weakrefs instead: new

Second issue exposed by fixing the first: the used_turfs list could contain turfs that weren't in an interior. Not an issue in most cases, but the sound system would check if a turf was in an interior then check which interior index it was. get_interior_by_coords() failed to find an interior and returned FALSE, which is equivalent to 0 and the sound system used that as the index and errored out: old
Fixed by returning null on failure instead: new

Third is the audio was now playing, but only in part of the interior. The interior's center wasn't being calculated correctly, and was being pinned in the top right corner: old
Fixed by correctly averaging the positions of the corners: new

Finally, the interior's dimensions were flipped, assigning the height to width and vice-versa: old
Corrected here: new

Explain why it's good for the game

Vehicle interiors being able to hear their own gunfire, movement sounds, as well as external fighting is cool and good.

Testing Photographs and Procedure

Changelog

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fix: vehicle interiors can hear exterior noises
/:cl: