Fix #10660: Sprite Font scale affected by viewport zoom level limits.

[PeterN]

Motivation / Problem

The maximum viewport zoom level setting inadvertently affects the scaled sprite font, since with variable interface scaling all the font_zoom related variables were merged with gui_zoom.

This issue does not affect scaleable fonts, only the built-in sprite font.

Description

Add separate _font_zoom which is only used to scale the sprite font. This is not clamped to the viewport zoom level limits, so always follows interface scale.

Limitations

Non-font sprites are still limited to the viewport zoom level, even when shown in a GUI. This is because there's currently no differentiation between viewport sprites and gui sprites, and also some sprites are used in both locations.

Could possibly be done but I think it's separate to this.

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

[rubidium42]

It's definitely a step in the right direction.