Fix 5e413c9d: Last sprite offset in GRF file was not recorded

[JGRennison]

Motivation / Problem

Last sprite offset in GRF file was not recorded.
This is noticeable as sprite 4626 (trees on road) from OpenGFX is missing on the title screen save.

Description

A line was missed when preparing PR #9988, as this had been added in a commit prior to the one which the PR was immediately based on.
This PR adds the missing line.

Limitations

N/A

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 touches english.txt or translations? Check the guidelines
• 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