Fixing bounding box errors

[PatrikLundell]

Summary

None

Purpose of change

• Change vehicle::bounding_box to produce what it says it should produce.
• Fix heli crash mx.
• Fix appliance merge usage of bounding box to work with the corrected version.

Describe the solution

• Change bounding_box to return the stated relative boundaries of the box, rather than the bubble coordinates of the same.
• Fix the heli crash mx logic to use the bounding box rather than just assume it's symmetric around the nominal position of the craft. This wasn't strictly necessary.
• Fix the appliance merge to compensate for the missing bubble coordinates.

Describe alternatives you've considered

Testing

Generated a bunch of heli crash mx, and they look reasonable.

The merging was harder to test, because the variables involved were optimized away (so I couldn't compare the original values before the change to those after). Tested by revealing a bunch of wires at a farming supply store and merged them by revealing the part in between them. However, that worked even before the bubble position compensation was added, so that doesn't show too much.

Additional context

The heli crash mx just happened to work because the default position of a vehicle object is the zero point, so the offsets become the only components of the resultant coordinates.