Fix: Desert/rainforest data coordinate 'y' is off by 1 when their 'x' coordinate is negative

[SamuXarick]

Motivation / Problem

The data that is used for generating desert or rainforest in sub-tropical is a list of TileIndexDiffC offsets, and visually look like a circumference.

The half towards NE looks misaligned.
It is similar to a bug that was uncovered back in 4846c7e for oil rig layout, in which it looked like this:

And once fixed, it looked like this:

This PR fixes the offset layout in the same manner as the oil rig layout.

Description

Make it aligned.

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

[LordAro]

Can you explain why you've changed the signs of the values, rather than just incrementing them like in the commit you linked?

[SamuXarick]

Can you explain why you've changed the signs of the values, rather than just incrementing them like in the commit you linked?

I was lazy, now it's incremented.

[LordAro]

Sure, why not