Fix a few wrong coordinate conversions that cause generation of meaningless overmaps

[park66665]

Summary

SUMMARY: Bugfixes "Fix a few wrong coordinate conversions that cause generation of meaningless overmaps"

Purpose of change

1. Playing a game for a long time
2. Meaningless overmap files, meaningless overmap files everywhere!
   
   

Describe the solution

Fix whatever that pops up while using gdb. I found and fixed two.
Another bug bites the dust

As this PR fixes wrongly used coordinate conversions, it will mend slightly(?) broken things.

Describe alternatives you've considered

1. Not doing this and go playing SS13 instead.
2. Not doing this and enjoying life playing A Hat in Time.
3. Restrict the power of generating overmaps that is given too easily. e.g. replacing overmapbuffer::get with overmapbuffer::get_existing. I AM LOOKING AT YOU, overmapbuffer::ter!

Testing

It compiles and passes test. I had no problems playing after the changes.

Additional context

This is capable of reducing future save folder's size by up to 30% or something.

[park66665]

Waiiiiit did I say duck-tape instead of duct-tape? Bruh, I guess VS Code doesn't spellcheck commit comments...

[va5h]

Waiiiiit did I say duck-tape instead of duct-tape? Bruh, I guess VS Code doesn't spellcheck commit comments...

Might solve a mystery
Or rewrite history!