Fix: Maximum space for engine preview image was never scaled.

[PeterN]

Motivation / Problem

Maximum vertical space for engine preview image is fixed at 40 pixels, regardless of interface scaling.

This has never been scaled, even before the recent changes.

Description

As this is drawing a sprite, this is resolved by scaling the maximum vertical space with ScaleSpriteTrad()

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 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