Fix #8168: allow relocating of HQ partial over existing HQ

[TrueBrain]

Motivation / Problem

Someone had an itch. It was small enough to scratch. What is there to say?

Description

Just in case you want to move it SLIGHTLY to the right.

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')
  • PR must be added to API tracker

[James103]

Again, the regression tests need to be updated.

[TrueBrain]

Again, the regression tests need to be updated.

Again, I know :) The CI is already telling me ;) Stop making me feel worse about doing my job poorly already :P

(PS: GitHub sends me mails when CI fails, so it is really hard to miss for me :D)