Fix #10151: Use smaller padding for viewport signs.

[PeterN]

Motivation / Problem

Before variable-scaling, the padding was always 1x1 pixel. This was changed to be scaled, except using the wrong dimension of 2x1 pixels instead of 1x1 pixel.

Description

This is resolved by using the fullbevel dimension instead of framerect when laying out viewport signs.
Note this only partially deals with #10151.

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