Fixes #8611

Motivation / Problem

I am not pointing fingers, but someone broke terraform limits when maxing out values :D

Additionally, when maxing out other values, you could constantly overflow the terraform limit, getting into a weird state of being allowed to terraform or not, depending on your game-tick.

Description

signed vs unsigned
uint32 + uint32 != uint64

C++ is making this very difficult for us :'(

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker