Fix: Wrong cargo line position in IndustryCargo window.

[PeterN]

Motivation / Problem

Cargo line position in industry cargo window was incorrect when many cargo types are present. This was noticed with the OTIS NewGRF at https://www.tt-forums.net/viewtopic.php?p=1242719#p1242719

Description

Resolved by changing calculation to determine the offset based on centring the cargo lines in the available space.

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

[PeterN]

Without this change:

With this change: