Let autotravel happen on other z-levels

[ghost]

Summary

SUMMARY: Bugfixes "Let autotravel happen on other z-levels"

Purpose of change

Fix autotravel not happening on other z-levels

Describe the solution

The path function added two tripoints together, but didnt account for the z coord, so it effectively doubled the z-coord when it wasnt 0, which made the path appear as a differnet z-level to players current position, so it bailed out.

Fixed this by manually amending the z-level of the combined tripoint afterwards.

Describe alternatives you've considered

N/A

Testing

Teleported to sewer, set autotravel route in overmap UI, it now works.

Additional context

N/A

[kevingranade]

This pull request has been mentioned on Cataclysm: Dark Days Ahead. There might be relevant details there:

https://discourse.cataclysmdda.org/t/cannot-confirm-overmap-autotravel/22102/6